U.S. patent number 4,619,859 [Application Number 06/654,149] was granted by the patent office on 1986-10-28 for highly-oriented stretchable multilayer film and process for producing the same.
This patent grant is currently assigned to Asahi Kasei Kogyo Kabushiki Kaisha. Invention is credited to Hideo Hata, Isao Yoshimura.
United States Patent |
4,619,859 |
Yoshimura , et al. |
October 28, 1986 |
Highly-oriented stretchable multilayer film and process for
producing the same
Abstract
The present invention provides primarily an entirely new type
multilayer oriented film suited for stretch wrapping, particularly
for wrapping of foods. Its features include various excellent
characteristics such as mechanical strength, transparency, heat
resistance, sealability, stretchability, low temperature
resistance, etc. In particular, the present invention concerns a
process for producing such a multilayer film by cold stretching a
multilayer containing at least one layer with a specific
composition capable of a high degree of cold stretching, which
enables simultaneously cold stretching of other layers made of
resins which can be or cannot be stretched solely, through its cold
stretching force, thereby imparting high strength to all the
layers, and further applying a specific heat treatment on the
stretched film to modify orientation of the film so as to increase
stretchability and to stabilize dimension of the film.
Inventors: |
Yoshimura; Isao (Fujisawa,
JP), Hata; Hideo (Yokohama, JP) |
Assignee: |
Asahi Kasei Kogyo Kabushiki
Kaisha (Osaka, JP)
|
Family
ID: |
16199760 |
Appl.
No.: |
06/654,149 |
Filed: |
September 25, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Oct 7, 1983 [JP] |
|
|
58-187079 |
|
Current U.S.
Class: |
428/213; 428/500;
428/518; 428/522; 428/349; 428/516; 428/520; 428/910 |
Current CPC
Class: |
B32B
27/32 (20130101); B32B 27/08 (20130101); B32B
27/22 (20130101); B32B 27/18 (20130101); B32B
27/322 (20130101); Y10T 428/2495 (20150115); Y10S
428/91 (20130101); Y10T 428/3192 (20150401); Y10T
428/31913 (20150401); Y10T 428/31855 (20150401); Y10T
428/31928 (20150401); Y10T 428/31935 (20150401); Y10T
428/2826 (20150115); B32B 2307/514 (20130101); B32B
2250/05 (20130101) |
Current International
Class: |
B32B
27/32 (20060101); B32B 027/08 () |
Field of
Search: |
;428/213,518-522,516,910,349 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Buffalow; Edith
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch
Claims
We claim:
1. A multilayer oriented film excellent in sealability and
stretchability, having high strength, high elongation and at least
three layers comprising:
(1) a base layer (M layer) containing primarily a specific mixed
composition selected from the group of polymer compositions
consisting of (A)+(B)+(C), (A)+(B) and (B)+(C);
(A) being at least one ethylene type polymer selected from a low
density polyethylene, a copolymer of ethylene with a monomer
selected from the group consisting of a vinyl ester monomer, an
aliphatic unsaturated mono-carboxylic acid, and an alkyl ester of
said mono-carboxylic acid, and the derivatives of said
copolymer;
(B) being a soft elastomer having a Vicat softening point of
60.degree. C. or lower;
(C) being at least one polymer having a Vicat softening point of at
least 100.degree. C. selected from crystalline polypropylenes and
crystalline polybutenes-1,
at least one M layer being provided adjacent to a core layer;
(2) an inside core layer (H layer) comprising primarily a polymer
selected from (C); and
(3) a surface layer (S layer) containing at least one polymer
selected from polymer (A), soft elastomer (B), crystalline
1,2-polybutadiene and a soft ionomer resin from an ethylenic
copolymer;
said film having a stress on 100% elongation from 100 to 600
(g/cm-width) on average in the longitudinal and traverse
directions.
2. The multilayer oriented film according to claim 1, wherein the
polymer (A) comprises at least one selected from the group
consisting of ethylene-vinyl acetate copolymers, ethylene-acrylate
copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylate
copolymers, ethylene-methacrylic acid copolymers and ethylenic
copolymer ionomer resins.
3. The multilayer oriented film according to claim 1, wherein the
soft elastomer (B) is a copolymer elastomer comprising 90 to 20
mole % of ethylene and at least one .alpha.-olefin selected from
the group consisting of .alpha.-olefins having 3 to 12 carbon
atoms.
4. The multilayer oriented film according to claim 1, wherein the
soft elastomer (B) is a copolymer elastomer having a Vicat
softening point of 50.degree. C. or lower and a crystallinity of
30% or lower.
5. The multilayer oriented film according to claim 1, wherein the
soft elastomer (B) is a copolymer further containing a small amount
of a polyene copolymerized therein in addition to ethylene and
.alpha.-olefin.
6. The multilayer oriented film according to claim 1, wherein the
soft elastomer (B) is a thermoplastic copolymer elastomer having a
melt index of 0.1 to 10 and the .alpha.-olefin in the soft
elastomer (B) is selected from propylene and butene-1.
7. The multilayer oriented film according to claim 1, having a
mixed composition with the ratios of respective components
satisfying the following relationships:
8. The multilayer oriented film according to claim 1, wherein the
base layer (M layer) comprises a mixed composition of
(A)+(B)+(C).
9. The multilayer oriented film according to claim 1, wherein the
base layer (M layer) contains 1 to 30 wt. % of a carrier (AS agent)
in addition to (A)+(B)+(C).
10. The multilayer oriented film according to claim 9, wherein the
carrier (AS agent) is at least one selected from the group
consisting of alicyclic saturated hydrocarbon resins, rosins,
petroleum resins, terpene resins, cumarone resins or modified
products thereof atactic polypropylenes (APP), 1,2-polybutadiene,
and ethylene-vinyl acetate copolymers with high vinyl acetate
content.
11. The multilayer oriented film according to claim 1, wherein the
base layer contains at least one anti-fogging agent, plasticizer or
mixtures there of in a total amount of 1 to 10 wt. %.
12. The multilayer oriented film according to claim 1, wherein the
base layer (M layer) comprises 20 to 90%, the skin layer (S layer)
5 to 40% and the core layer (H layer) 5 to 40% of the total
thickness, respectively.
13. The multilayer oriented film according to claim 1, having at
least the layered structure of S layer/M layer/H layer/M layer/S
layer.
14. The multilayer oriented film according to claim 1, having at
least the layered structure of S layer/H layer/M layer/H layer/S
layer.
15. The multilayer oriented film according to claim 1, having a
stress on 200% elongation of 200 to 1,000 g/cm-width on average in
the longitudinal and transverse directions.
16. The multilayer oriented film according to claim 1, wherein (A)
is a copolymer of ethylene having a vinyl ester monomer and (C) is
a crystalline polypropylene.
Description
BACKGROUND OF THE INVENTION
This invention relates to a multilayer type orientation-modified
highly stretched film to be provided for use primarily in wrapping
material. More particularly, it pertains to a multilayer film
excellent in heat resistance, sealability as well as
stretchability, comprising a layer structure of at least three
layers, with at least one layer containing a specific mixed
composition (M layer), a layer comprising at least one polymer
selected from crystalline polypropylene and crystalline
polybutene-1 (H layer), and a surface layer (S layer), wherein the
whole layers are stretched in at least one direction to be highly
oriented, followed subsequently by application of a specific heat
treatment to modify its orientation.
The film of the invention is useful for various wrapping purposes
such as stretch wrapping, stretch-shrink wrapping, shrink wrapping,
etc. or otherwise for skin packing, cohesive wrapping or other
wrapping or packaging uses. While the invention is not particularly
limited in its use, it is particularly suitable for stretch
wrapping and stretch-shrink wrapping, and the following description
is made primarily by referring to these uses as an embodiment of
practical applications of the present invention.
Packages formed with films are manufactured by a good number of
methods which utilize to advantage the characteristics of the films
such as the bag sealing method, twist wrapping method, thermal
shrink wrapping method, cohesive wrapping method by use of specific
films represented by Saran Wrap (product by Asahi Kasei Kogyo
K.K.), stretch wrapping method, skin packing method, etc. These
methods require respective wrapping characteristics. For each
packaging method, therefore, it is generally practiced under the
present situation to select a film whose basic material,
composition, form and characteristic attributes best suit the
wrapping characteristics of the particular method employed.
The shrink wrapping method, on the principle of full use of the
heat shrink property of a film which has been stretched to acquire
a specifically set orientation, comprises the steps of loosely
pre-wrapping and sealing a given article subjected to wrapping so
as to enclose the article with the film and, thereafter exposing
the film to a heat medium such as a current of hot air, an infrared
ray, or hot water and the like, thereby causing the film to shrink
and come into tight contact with the overall contour of the
article. This method is characterized by the fact that the produced
package has a beautiful appearance which adds to the commodity
value of the wrapped article. In addition, the package keeps the
contents hygienic, permits shoppers to touch and usually examine
the quality of the contents, keeps the contents tightly in position
regardless of shape or whether there are a plurality of pieces in
one package, and provides the content with ample protection against
vibrations and impacts. Compared with the stretched wrapping method
which is used extensively in supermarkets, the shrink wrapping
method provides high speed wrapping of the article. Also, this
method permits the wrapping of an article with a complicated shape
which cannot be wrapped by the stretch wrapping method and also,
the wrapping of an article without a vessel such as a tray. While
there is another advantage enabling tighter wrapping, there is
involved the disadvantage that the film must be heated sufficiently
until the film is shrunk. On the other hand, the shrink films
generally employed in the art are not suited for the purpose of
stretch wrapping because of drawbacks such as small elongation of
the film itself, breaking of the film when overly stretched, too
strong a stress relative elongation to effect elongation easily and
substantial absence of self-stickiness. These are the drawbacks and
problems of the shrink films of the prior art.
On the other hand, as is well known, stretch wrapping has been
popularly employed in recent years in super markets for wrapping
vegetables, fruits, fresh feedstuffs, meat and various cooked
foods. The advantages are that is it high in fitness capable of
responding to uneven shapes or sizes of the article to be wrapped
without causing the film to have residual permanent deformation or
creases when stretching the readily elongated film. Also, the film
can be easily fixed by light pressure or heat setting of the drawn
film without sagging of the packages. In addition, it has an
appropriate air permeability for fresh foods thereby preserving
freshness and preventing a reduction in weight of the article to be
wrapped. Furthermore it can keep the article to be wrapped hygienic
and permits shoppers to confirm the quality by vision and touch.
The wrapped article is also beautiful in appearance thereby adding
greatly to the commodity value of the article to be wrapped. Both
the inexpensive manual wrapping machine and the highly efficient
automatic machine are easily available and no heat is given to the
article to be wrapped.
In the prior art, however because of employing an unstretched film
having a high elongation of the film, there are involved numerous
problems. The mechanical strength of the film is low resulting in
frequent breaking of the film or formation of holes when sealing
the folded films at the bottom of a tray. The operability
(mechanical suitability, manual workability) is poor due to a low
modulus of the film, whereby the film cannot be made extremely thin
in view of its quality, and thus thick films must be used.
Heretofore, with an aim to develop a film having properties equal
to or better than those of the plasiticized polyvinyl chloride film
(containing 30 to 33 wt. % of plasticizer), various manufactures
have developed films comprising various resins (e.g. EVA;
ethylene-vinyl acetate copolymer, 1,2-polybutadiene resin,
L-LDPE:linear LDPE, etc.), which are substantially unoriented films
consisting of other kinds of resins, to challenge the market.
However, under the present situation, none of these attempts are
considered to be successful. Further, while development of a
complete film other than plasticized polyvinyl chloride film is
rapidly desirable because of the problems in pollution and hygiene
at the present time, only an insufficient film which is poor in
handling is now being used as a trial. Such a film cannot be
handled with ease and is therefore not welcomed at the working site
of wrapping. The amount of such a film employed is very small and
far from providing a substitute for the polyvinyl chloride film.
These films are those of high quality to be utilized primarily for
foods, as distinguished from those films having low levels of the
requisite characteristics to be generally utilized for industrial
applications, such as pallet stretch film.
The manufacture of a drawn film of polypropylene on the other hand,
is accomplished by a method comprising the steps of melt extruding
the polymer resin through an extruder die, quenching the extruded
tubular sheet, reheating the cooled tubular sheet at a high
temperature within the range of e.g. from 150.degree. to
160.degree. C. , and forcing air into the inner cavity of the
tubular sheet. In the case of a drawn film of low-density
polyethylene, a similarly extruded tubular sheet of the polymer
resin is biaxially drawn in an effort to set a high degree of
molecular orientation in the film. In the course of the drawing,
however, the sheet bursts, making a manufacture of film hardly
practicable from the technical point of view.
Because of the above mentioned difficulty, the generally method is
a direct inflation method which comprises the steps of extruding
the polymer resin at a temperature within the range of from
180.degree. to 220.degree. C., for example, and subsequently
causing the extruded sheet, by means of a proper form of air, to be
simultaneously cooled and inflated to a prescribed size.
While this method is characterized by producing a film easily at
low cost, it is impossible to set satisfactory molecular
orientation by stretching, because flowing will readily occur
between molecules. Also, the optical properties are very inferior.
Accordingly, for use in shrink wrapping, the resultant film has low
heat shrink percentage, low heat shrink stress and shrinks at
rather high temperatures. Therefore, such a film can be used only
in special uses with increased film thickness. For the purpose of
improving the above drawbacks, it has been proposed to prepare a
film of low density polyethylene according to the steps of
extruding a film of low density polyethylene, irradiating the
resultant film with a high energy radiation to effect partial
crosslinking thereof, stretching the film by heating to a high
temperature (e.g. 140.degree. C.) in excess of the melting point,
whereby flowing between molecules can be prevented to sufficiently
set molecular orientation. However, the extent of low temperature
shrinkability, which is one of the most important characteristics
in the shrink packaging method, is low and the film obtained
readily bursts with small elongation. The film produced according
to this process is small in elongation and too high in stress on
elongation to 100 or 200% (frequently not elongated to such
percentages). Therefore, it is a film which cannot entirely be
utilized for stretch wrapping film, and is only utilized in a part
of shrink wrapping as a film shrinkable at high temperature.
As a new category of wrapping films, a variety of composite
multilayer films have been known.
Recently, to meet the demand of higher requisite characteristics,
composite (multilayer) films have been increasingly developed. For
example, there exist generally a large number of composite films
having other resins laminated by melting on unoriented films or
oriented films.
For example, various kinds of films and combinations are chosen
depending on uses, such as a film improved in heat sealing property
by fusion lamination of other resins on an unoriented polypropylene
according to the casting method (called as C.PP) or an oriented
propylene (O.PP) or a film improved in barrier performance prepared
by coating with a polyvinylidene chloride type latex (called as K
coat film), etc.
On the other hand, it has also generally been known in the art to
produce a co-extruded film of unoriented type by melting various
kinds of resins separately in respective extruders, permitting the
molten resins to be confluent within a multilayer die to be
combined and extruded therethrough, followed by cooling, into films
and sheets.
However, in the case of a stretched multilayer film, in the first
place, each layer constituting the multilayer product differs in
optimum extruding conditions, stretching conditions, etc. depending
on the resin employed, even if it is desired to obtain a film
stretched to a high degree. According to the technique of the prior
art, unfavorable phenomena occur such as uneven thickness, vertical
streak, puncture, bursting, peel-off of respective layers and
whitening due to interface roughening occurred, whereby no
satisfactory product can be obtained. Also, even when a small piece
of the film may be obtained, such a product is far from the film
having intended characteristics under the present situation. It has
been deemed to be very difficult to solve these drawbackes.
Moreover, it has been deemed to be entirely impossible to use an
oriented film as a stretchable film which requires stretchability
and no such thought has ever been conceived. The present inventors
have invented U.S. Pat. Nos. 4,277,578, 4,399,181 and 4,430,378 to
overcome the drawbacks of the shrink films as described above. The
films proposed by these patents are more suited for uses of shrink
wrapping. Further, as a film more suited for uses of stretch-shrink
wrapping, the present inventors have invented a film as disclosed
in Japanese laid-open Patent Publication No. 175,635/1983. But that
film was found to be yet unsatsifactory as a complete stretch film,
with respect to its stress balance relative to elongation and also
elongation as a whole. Thus, in some cases, it has been frequently
utilized primarily with respect to its low temperature shrinkable
properties acquired from the cold stretching method, secondarily
with less stress imposed on elongation. In other words, the film
was utilized as a film having more excellent elongation and low
temperature shrinkability as compared with the films of other
methods. The present invention has achieved a still more excellent
invention which satisfies sufficiently the properties deficient in
these inventions and also enlarged its uses. This will be clarified
in more detail by comparison with Comparative examples as
hereinafter described.
As a more detailed description of the stretch wrapping method, the
films to be used commercially for these methods of the prior art
are only films made of materials containing a large amount of
plasticizers, such as soft polyvinyl chloride resins containing as
much as 30 wt. % or more of plasticizers (hereinafter abbreviated
as PVC). These films cannot be extruded or endowed with a flexible
property to be unsuitable for this kind of use (stretch), unless a
large amount of a plasticizer is mixed therein. Thus, since a
larger amount of a plasticizer, for example, dioctylphthalate,
dioctyladipate, etc. is employed, there are involved various
problems such that the amount of water vapor permeated through the
films is increased to readily cause denaturation of the article to
be wrapped. Moreover the plasticizer is liable to migrate to the
article to be wrapped and cause contamination thereof. Cutting of
the film by hot wire during wrapping working will cause generation
of the gas of the plasticizer and corrosive chlorine type gas which
are undesirable from viewpoint of hygiene; and toxic gases are
generated during incineration of the used films. Finally, the film
is inferior in cold resistance and is less pliable, brittle, liable
to burst during storage of the wrapped product at low temperatures
and the film roll is heavy.
Whereas films constituted only of high density polyethylenes, low
density polyethylenes or polypropylene polymers among polyolefins
for general purpose have excellent properties with respect to the
drawbacks in pollution as described above, they are devoid of
important properties necessary for uses as intended by the present
invention. Thus, it has been difficult to provide a practical film
for stretch wrapping capable of satisfying the various
characteristics as mentioned below with the use of these films.
More specifically, the film to be used for stretch wrapping must,
at the same time, satisfy all the characteristics as mentioned
below. Thus the films must have:
(a) adequate stress relative to an adequate elongation, and also a
high elongation at break;
(b) excellent film-to-film adhesion;
(c) adequate delayed recovery characteristic, high deformation
recovery, an adequate elastic elongation, and strong in mechanical
strength;
(d) adequate slipping characteristic at the surface of a film;
(e) excellent optical characteristics such as transparency and
gloss;
(f) adequate gas permeability;
(g) excellent anti-fogging property without retention of water
droplets on the surface;
(h) excellent wrapping operability; and
(i) heat resistance during sealing.
For example, when an unoriented film of polypropylene is first
drawn for stretch wrapping, only a certain portion is elongated,
whereby the phenomenon called necking occurs with generation of
extreme thickness irregularity. Even after removal of the load, the
portion remains in the state as drawn, whereby the wrapped product
is greatly damaged in appearance and the intended purpose of
wrapping is not achieved. Also, the stretched film is hard, strong
and small in elongation, requiring a very great force for
elongation up to the point at which the article to be wrapped is
broken. Such a film has no tackiness and, even if a plasticizer
such as a liquid polybutene or a polybutene with low polymerization
degree may be mixed therein, no tackiness can be imparted thereto,
unless it is used in an amount of 5 wt. % or higher. If employed in
such an amount, since polyolefins lack the ability of retaining
plasticizers as possessed by polyvinyl chloride resins, most of the
plasticizers will bleed out on the surface to make the film sticky
and practically unuseful.
High density polyethylene films are also similarly hard and cannot
easily be stretched to give similar results. In fact, they are
opaque without luster, thus having no possibility of
application.
Low density polyethylene films are relatively softer as compared
with those as mentioned above, but non-stretched films suffer also
from necking, with the deformation recovery being small, having
lower strength, no good transparency and no tackiness. Thus, they
are not useful for the purpose of the invention. On the other hand,
low density polyethylene films crosslinked with, for example, an
electron beam so as to be made readily stretchable, and stretched
according to a conventional method, have the same drawbacks as
polypropylene, and therefore they also are not provided with
properties suitable as the base material for stretch wrapping, as
intended by the prevent invention.
In the case of an elastic elastomer having substantially complete
deformation recovery comprising a base material such as
styrene-butadiene latex or other rubbers, although there is no such
phenomenon as necking, there is a problem with respect to optical
characteristics and hygiene of foods. In addition, the strength
when elongated is approximately in direct proportion to the
elongation and the response of relaxation of deformation recovery
is effected momentarily. That is the film will be restored to its
original state momentarily when it is released in setting the film
end portion at the article to be wrapped or at the bottom portion
of a tray, whereby the wrapping will be loosened. Thus, this kind
of material does not have the desired properties suitable for one
of uses intended by the present invention.
Of these polyolefin type films, there are also commercially sold
trial samples comprising mainly crystalline 1,2-polybutadiene type
or ethylene-vinyl acetate copolymer (EVA), admixed with an
anti-fogging agent, tackfier, etc., formed into films in a
conventional manner (T-die method, air-cooling inflation method,
etc.). However, these films have a number of drawbacks and have not
yet commercially been sold in full-scale without reaching the level
as substitute for films of the prior art.
The above films cannot eliminate at the same time all of the
properties antagonistic to each other such as easy elongation
during packaging, heat resistance at the sealed portion, easy
sealing, and, further, prevention of bursting during packaging due
to deficiency in film strength. Thus, the films consequently have
only average and incomplete properties. For example, in the case of
an EVA type film, when the content of vinyl acetate (VAc) in EVA is
increased, the film turned around the bottom portion of a vessel
will tend to be molten by heat and burst readily. For this reason,
in order to prevent such a tendency, the thickness of the film must
be increased from 16.mu. to 20.mu., or 22.mu. or 24.mu.. As a
consequence, drawbacks occur such that the film becomes hardly
elongatable, the films become less sticky before sealing due to the
rubbery elastic components and the increased film thickness makes
sealing more difficult. Moreover, there is also involved the
problem of disadvantageous increase of cost. Then, as a next
measure, there is adopted the method of incorporating, for example,
a low density polyethylene (particularly of the linear type), a
polypropylene or a rubber into EVA. By such a method, however, no
great extent of improvement can be brought about. On the contrary
there is the tendency that important characteristics such as
transparency, gloss, etc. may be lowered to further pose additional
problems.
SUMMARY OF THE INVENTION
The present inventors have studied to further improve the drawbacks
of these films and preparation methods, and consequently found a
film having effective stretchability and being improved to a great
extent also in additive properties such as heat shrinkage at low
temperature, broadness in dependency on temperature of heat
shrinkage characteristic, optical properties, low vapor
permeability, sealability of the film, tensile strength and
elongation at break, and low temperature flexibility. The film is
not inferior to plasticized PVC films but is more excellent in
various characteristics. The film is also improved due to a
specific process for the preparation thereof which is not expensive
and excellent in processability.
More specifically, the present inventors have accomplished a novel
film and a process for the preparation thereof by arranging
specific layers and forming them into a film by treatment under
specific processing conditions, whereby a film not found in the art
having eliminated the various defects as described above at the
same time, which is suitable widely for various kinds of wrapping
methods, particularly as stretch wrapping film, can be obtained. In
other words, the present invention concerns a composite film and
its specific feature resides in provision of layers comprising
specific mixed compositions or components further in combination
with other specific layers as well as the processing of cold
stretching under specific conditions, whereby a high degree of
stretched orientation and other excellent characteristics not found
in the prior art can be exhibited through the synergistic effect of
said mixed compositions or components with the layer of other kinds
of resins, and further the orientation can be modified by
application of a specific heat treatment. For example, through the
synergistic effect of its treatment effect and the composition,
without lowering significantly the tensile strength of the S--S
curve, the stress at the middle portion, namely the portion
necessary for stretch wrapping, can be lowered to be more easily
elongated. In particular, under the conditions which cannot stretch
in case of an individual layer, various characteristics,
particularly high degree of stretched orientation can be imparted
very stably to respective layers, whereby a multi-layer film
particularly excellent in various characteristics such as
elongtion, strength, transparency, etc. can be obtained. The film
of the present invention is not limited to the film for stretch
wrapping but useful as films for various kinds of stretching. As
another application, it can also provide a film having good
properties such as a shrink film, particularly a film excellent in
optical characteristics, strength, heat seal characteristic,
elongation, stress relaxation characteristic as well as low
temperature shrinkage characteristic, shrinkage response (speed),
etc. As still another application, it is a film suitable for
stretch-shrink wrapping.
More particularly, the present invention concerns a multilayer
oriented film excellent in sealability and stretchability, having
high strength and high elongation, which comprises at least three
layers of:
(1) a base layer (M layer) containing primarily a specific mixed
composition selected from the group of polymer compositions
consisting of (A)+(B)+(C), (A)+(B) and (B)+(C),
(A) being at least one polymer selected from a low density
polyethylene, or a copolymer of ethylene with a monomer selected
from a vinyl ester monomer, an aliphatic unsaturated
mono-carboxylic acid or an alkyl ester of said mono-carboxylic acid
the derivatives of said copolymer;
(B) being a soft elastomer having a Vicat softening point of
60.degree. C. or lower;
(C) being at least one polymer selected from crystalline
polypropylenes and crystalline polybutenes-1,
at least one such M layer being provided adjacent to the core layer
as defined below,
(2) a core layer (H layer) as inside layer comprising primarily a
polymer selected from (C); and
(3) a surface layer (S layer) containing at least one polymer
selected from said polymer (A), soft elastomer (B), crystalline
1,2-polybutadiene or a soft ionomer resin from an ethylenic
copolymer,
said film having a stress on 100% elongation of from 100 to 600
(g/cm-width) on an average value in the longitudinal and transverse
directions.
DETAILED DESCRIPTION OF THE INVENTION
The base layer (M layer) comprises specifically a mixed composition
of (A) and (B), (B) and (C), or (A), (B) and (C), wherein (A) is at
least one polymer selected from a low density polyethylene, or a
copolymer of ethylene with a monomer slected from a vinyl ester
monomer, an aliphatic unsaturated mono-carboxylic acid or an alkyl
ester of said mono-carboxylic acid the derivatives of said
copolymer; (B) is a soft elastomer having a Vicat softening point
of 60.degree. C. or lower; and (C) is at least one polymer selected
from crystalline polypropylenes and crystalline polybutenes-1.
Further, said composition may also contain a composition comprising
primarily a component subjected to energy ray treatment, having an
insoluble gel in boiling xylene of 0 to 50 wt. % and a melt index
of 1.0 or less. In the prior art, for improvement of various kinds
of characteristics of a crystalline polypropylene (hereinafter
abbreviated as IPP), particularly heat stability, strength at low
temperature and impact resistance, it has been proposed to prepare
a composition comprising primarily IPP, which is mixed partially
with an ethylene-propylene copolymer (hereinafter abbreviated as
EPR), as disclosed in Japanese Patent Publication No. 7088,
Japanese Patent Publication No. 15042/1961 and Japanese Laid-open
Patent Publication No. 78977/1977. However, according to these
methods, compatability between IPP and EPR is not necessarily good,
and therefore, when worked into a thin film, the surface of the
film was roughened and thereby worsened greatly in optical
characteristics and its strength was insufficient. It has also been
proposed, in order to improve such drawbacks even to a small
extent, to further mix an atactic polypropylene into the
composition, as disclosed in Japanese Laid-open Patent Publication
Nos. 112946/1974 and 96638/1973. Each of these improvements is
based primarily on IPP, but the problems still remain with respect
to kneading dispersibility, strength, heat resistance, sealability,
etc., when the composition is formed into a thin film.
The components of a specific composition to be used in the present
invention synergistically improve heat sealability, various
strength characteristics, flexibility, transparency, tensile
modulus, heat durability, cold durability, adhesion to other
layers, etc. They cannot only be controlled to give a film which
ranges from a relatively soft film to a rigid film, but they also
enable cold stretching of the other resin layer, which cannot be
stretched at all as a single layer, under the conditions of the
present invention during working of the film, thus providing
synergistic improvement and a particularly excellent film.
The composition (A) employed here for the base layer or other
layers should preferably be a moderately soft polymer with
relatively low crystallinity (20 to 65% of crystallinity according
to the X-ray method), which is at least one polymer selected from
LDPE (preferably a linear low density polyethylene), or a copolymer
of ethylene with vinyl ester monomers, aliphatic unsaturated
mono-carboxyic acids or derivatives of said copolymer. These
copolymers include preferably ethylene-vinyl acetate copolymer
(EVA), ethylene-ethyl acrylate copolymer (EEA), ethylene-methyl
methacrylate copolymer (EMMA), ethylene-acrylic acid copolymer
(EAA), ethylenemethacrylic acid copolmer (EMA) or these copolymers
of which at least a part of the polymer having carboxyl groups
saponified at least partially is converted into an ionomer (ionomer
resins), and the content of the monomers other than ethylene should
preferably be 2 to 12 mole %, more preferably 3 to 10 mole %. If
the content is 2% or higher, sealability, flexibility, transparency
and various strength characteristics become excellent. At a level
higher than 12 mole %, extrusion moldability, mixing with other
components or heat resistance may become inferior, or when the
copolymer is provided as the surface layer to be an outer layer,
blocking may occur between the surfaces, whereby a problem with
respect to handling tends to arise. The resin selected from these
starting materials and used as such may have a melt index generally
of 0.2 to 10, preferably 0.3 to 5. With a melt index less than 0.2,
there are problems with respect to mixing and extrudability of the
starting material. On the other hand, above the upper limit, the
various strengths may sometimes be insufficient. For example,
during stretching, there may occur unfavorable phenomenon such that
the bubble may readily be broken. Among them, the most preferable
material to be used for the mixed composition layer (base layer) is
EVA, with the vinyl acetate content (hereinafter abbreviated as
VAC) being preferably 3 to 8 mole %, more preferably 3 to 7 mole %.
The linear low density polyethylene (L-LDPE) refers to a linear low
density polyethylene obtained by the medium pressure, low pressure
or, in some cases, high pressure method. Particularly, it contains
as the .alpha.-olefin 7 mole % or less, preferably 1 to 5 mole %,
of at least one member selected from .alpha.-olefins having C.sub.3
-C.sub.12 carbon atoms such as propylene, butene, pentene, hexene,
heptene, octene, 4-methyl-1-pentene, etc., copolymerized therein.
Otherwise, it may also be one containing a monomer having a polar
functional group such as vinyl ester (e.g. vinyl acetate)
copolymerized therein. They should preferably have a melt index
(abbreviated as MI) of 0.2 to 10 and a density of 0.910 to 0.935
g/cm.sup.3. They should also have a crystal melting point (m.p.)
within the range of from 110.degree. to 125.degree. C. according to
the DSC method (measured at a scan speed of 10.degree. C./min.),
and are distinguished from the branched low density polyethylenes
having densities of 0.915 to 0.925 g/cm.sup.3 and crystal melting
points of 100.degree. to 108.degree. C.
Referring next to the soft elastomer having a Vicat softening point
of 60.degree. C. of the component (B), the component includes
ethylene-.alpha.-olefin copolymer elastomer, butyl rubber elastomer
and styrene-butadiene elastomer (particularly thermoplastic block
copolymer), etc. Preferably, it is a thermoplastic elastomer
comprising ethylene and an .alpha.-olefin, which is a copolymer of
ethylene and at least one .alpha.-olefin selected from
.alpha.-olefins having 3 to 12 carbon atoms, which copolymer may
also contain a small amount of a hydrocarbon having a polyene
structure such as dicyclopentadiene, 1,4-hexadiene,
ethylidenenorbornene or others copolymerized therein. Examples of
.alpha.-olefin are propylene, butene-1, hexene-1, heptene-1,
4-methyl-1-pentene, octene-1, etc., preferably propylene and
butene-1. The content of ethylene in the copolymer may be 20 to 95
mole %, preferably 40 to 93 mole %, more preferably 65 to 90 mole
%. Further preferably, it is 75 to 85 mole %.
With respect to the properties of these polymers, they should have
a density of 0.910 g/cm.sup.3 or lower. When they are to be used as
the mixed composition base layer, they should preferably have a
Vicat softening point [ASTM D1525 (value under load of 1 Kg)] of
60.degree. C. or lower, more preferably 50.degree. C. or lower,
including generally from those substantially amorphous at the
rubbery region to partially crystalline polymers of low
crystallinity of about 30% according to the X-ray method or
partially crosslinked polymers. Preferable is the case of a
copolymer of ethylene with propylene or butene-1, which may further
contain a small amount of a compound having a diene structure as
the comonomer. For example, there may be included thermoplastic
elastomers such as random copolymers polymerized with a catalyst
system of a vanadium compound and an organic aluminum compound,
having a melt index of 0.1 to 10, preferably 0.2 to 6. These are
preferably supplied according to conventional handling in the form
of pellets, as different from non-vulcanized rubber in general
having a shape of a block, and yet free from cold flow, and should
preferably have sufficient thermoplasticity capable of being
extrusion molded simply by extruding a blend prepared by dry
blending. These are more preferable than the elastomers referred to
as EPM or EPDM which are produced and commercially available
generally in the form of a great block with its viscosity being
expressed by Mooney viscosity, from which a mass is cut off,
thoroughly kneaded with other thermoplastic polymers in a bambury
mixer and cut into pellets. They can also be modified by effecting
a small amount of crosslinking. Also, a monomer having a polar
functional group (e.g. carbocylic acid) may also be added for
modification of these elastomers.
The polymer (C) comprises a component which is relatively rigid and
has a relatively high crystallinity, and refers to at least one
selected from crystalline polypropylenes and high molecular weight
polybutene-1 (hereinafter abbreviated as IPP and PB-1,
respectively). These may preferably comprise relatively rigid
polmers having Vicat softening points of 100.degree. C. or higher.
IPP, which is one of the polymers (C), refers to a crystalline
polypropylene having high isotacticity such as those commercially
sold, preferably those containing homopolymers of propylene or
copolymers of propylene and 7 mole % lower of other .alpha.-olefins
such as ethylene, butene-1, etc. Alternatively, they can also be
freely mixed.
The polymer (C) has a melt flow rate (hereinafter abbreviated as
MFR) of 0.1 to 30, preferably 0.5 to 20, more preferably 0.7 to 15.
If the melt flow rate is less than 0.1, problems may occur in
mixing during processing or a haze may result in the resultant
film. With a melt flow rate higher than the upper limit, problems
may occur with respect to extrusion stability and stability at the
sealed portion.
Polybutene-1 should preferably have a crystalline structure with a
high molecular weight containing 93 mole % or more of butene-1,
which may also include copolymers with other monomers, with a melt
index of 0.2 to 10, for the same reason as described above, as
different from liquid or waxy polymers of low molecular weight. It
is also preferred to use a mixture of IPP and PB-1 as described
above. Also, in addition to those, if there is a rigid polymer
suited for the object of the invention having adequate
compatibility and dispersibility, it can also be used in the
present invention. When the polymer (C) is used as the H layer as
hereinafter described, it may be used as a mixture with up to 50
wt. % of other polymers [also at least one selected from the
polymer (A) and the polymer (B) as described above].
The base layer comprising the specific mixed composition or
components is constituted of the respective components as describe
above, and its combination and mixed amounts consist substantially
of (1): (A) and (B), (2): (B) and (C) or (3): (A), (B) and (C),
with the ranges of their amounts being preferably:
(1) 0.05.ltoreq.B/(A+B).ltoreq.0.90
(2) 0.30.ltoreq.B/(B+C).ltoreq.0.90 or
(3) 0.05.ltoreq.B/(A+B).ltoreq.0.90 and
0.05.ltoreq.C/(A+B).ltoreq.2.0,
more preferably,
(1) 0.07.ltoreq.B/(A+B).ltoreq.0.70
(2) 0.40.ltoreq.B/(B+C).ltoreq.0.87 or
(3) 0.07.ltoreq.B/(A+B).ltoreq.0.70 and
0.07.ltoreq.C/(A+B).ltoreq.1.0,
further preferably,
(1) 0.10.ltoreq.B/(A+B).ltoreq.0.50
(2) 0.50.ltoreq.B/(B+C).ltoreq.0.85 or
(3) 0.10.ltoreq.B/(A+B).ltoreq.0.50 and
0.10.ltoreq.C/(A+B).ltoreq.0.40
Here, when the amount of the soft component (B) mixed is too small,
in all the cases of (1), (2) and (3), the synergistic effect as a
mixture can hardly be exhibited to lower various characteristics.
For example, strength of the film, haze value, low temperature
characteristics, flexibility, sealability, cold stretching, etc.
will become inferior. On the contrary, if it is too much, the film
will excessively be softened, whereby heat resistance, seal
characteristic and haze value tend to be deteriorated. Also, as
shown stepwise in the above formulae, when the range of the
component (B) is restricted to narrower ranges, the synergistic
effect gradually becomes greater in either case of (1), (2) and
(3). For example, strength, haze value, flexibility, sealability
and stretchability during drawing of the film will be improved.
Among the respective combinations of the mixed compositions as
mentioned above, a particularly preferable combination is one
comprising primarily [(A), (B) and (C)] of (3). More preferably as
the base layer, 1 to 10 wt. % of at least one additive selected
from liquid or paste-like anti-fogging agents and plasticizers are
added to these combinations. Further preferably, in addition to
these, 1 to 30 wt. % of at least one carrier for holding these
additives as hereinafter described is contained in the composition.
The same tendency can also be seen in the case of the composition
of A+B or B+C.
With respect to the component (C), this component improves tensile
strength, impact strength, heat resistance, extrusion moldability,
modulus, heat seal range of the mixed composition synergistically
with other components, particularly great in effect in heat
resistance, extrusion moldability, modulus and heat seal range. If
the amount of the component is to small, the effect will be lowered
in, for example, workability, heat seal range and improvement of
strength of the film. Also, the expected value of heat resistance
will be lowered. If it is too much, extrusion moldability,
transparency, flexibility and impact strength will be undesirably
deteriorated. Therefore, the above range is preferred. Here, the
component (A) may preferably comprise a specific ethylenic
copolymer selected from those as described above, and it is
sometimes preferred to constitute the main component in the mixture
of the three components of (A), (B) and (C).
Of the three components, the component (A) and the component (C)
cannot usually be mixed well with poor compatibility, and the
synergistic effect described above cannot be expected for such a
mixture. But the drawback of such a mixture can be markedly
improved by addition of the component (B).
This may be considered to be due to complicated synergistic actions
such as the severe interaction between the characteristics derived
from the structure related to ethylene and the polar functional
group contained in the component (A), the crystalline structure of
the mixture and the dispersed state of the mixture, the effect of
the treatment, etc.
For example, in the case of the component (A) as the main
component, when the respective components as described above are
dry blended in pellets and extruded by fusion kneading through an
extruder excellent in kneading ability into a raw film, it is
possible to conceive a state where the component (B) is dispersed
complicatedly internally of the component (C) dispersed in the
component (A) or in the vicinity thereof, while undergoing reaction
or interaction with these components.
These can give different shapes depending on the molding
conditions, when a flow orientation is given by processing of a
film-shaped molding.
For example, when the above mixture is extruded through a small
slit, for example a die for film or sheet having a 1.5 mm
thickness, it can be extruded as such or with application of a
certain draw ratio and quenched to form a film. In the case of, for
example, mixing 20 wt. % of PP, some portions of the component (C)
and PP were oriented in the direction flowing through main
component, with the dispersed particles oriented in fibrous state,
although such an effect may differ depending on the rigid component
(C) employed and its amount. In some cases, the structure may
become as if reinforced with glass fibers to exhibit various
extremely improved characteristics such as strength.
The component (C), which can also be dispersed in a flat shape
depending on the working conditions to elongate the path length,
may be considered to have a barrier performance against the
aforesaid additives (anti-fogging agent, plasticizer), thereby
having the action of prolonging the effect of such additives by
controlling bleed-out onto the surface or the action of enhancing
the retaining effect.
On the other hand, in the case of (B)+(C), the thermoplastic
elastomer (B) particularly preferably used is an amorphous or a
partially low crystalline random copolymer of 65 to 95 mole %,
preferably 75 to 90 mole %, of ethylene with at least one
.alpha.-olefin selected from propylene and butene-1, which
copolymer is supplied in pellets and used by dry blending. The base
layer may also be used so that the aforesaid mixed polymer may
exist in an amount of at least 50 wt. %, preferably 80 wt. % or
more, within the range which does not impair various
characteristics. Also, the base layer contains other resins mixed
therein.
The specific mixed composition of the components to be employed for
the special multilayered film of the present invention may be
subjected to, after extrusion into a multilayer to provide a raw
film, activation treatment with a high energy ray, such as electron
beam (.beta.-ray), .gamma.-ray, or UV-ray for modification of the
polymer by effecting a crosslinking reaction. The extent of the
"crosslinking reaction" may be from 0 to 50 wt. % of insoluble gels
in boiling xylene and 1.0 of melt index, preferably from 0.1 to 40
wt. % of the same gels with a melt index of 0.5 or less,
more preferably from 0.5 to 30 wt. % of the same gels with a melt
index of 0.1 or less,
further preferably from 1 to 25 wt. % of the same gels with a melt
index of 0.1 or less,
still further peferably from 1 to 20 wt. % of the same gels with a
melt index of 0.1 or less.
If the insoluble gels are greater in amount than the range as
specified above, the molded product will be deteriorated in
elongation, strength, etc. In particularly, there ensue problems
when molded into films such that the film cannot be sealed, the
film cannot be cut with hot wire, the film is readily broken, etc.
Thus, the range as specified above is preferred. The amount of the
insoluble gels was shown stepwise in the above description, and the
balance of the characteristics such as sealability, heat
resistance, stretching, etc. can be improved as the range for the
above amount becomes narrower.
Next, as another layer of the present invention, the H layer
comprising the polymer (C) comprises primarily a homopolymer or a
mixed polymer selected from the crystalline polypropylene (IPP) and
the crystalline polybutene-1 (PB-1) as described above, preferably
the latter mixed polymer. In addition to the above, a mixture
containing 50 wt. % or lower, preferably 40 wt. % or lower, more
preferably 30 wt. % or lower, of other polymers having a lower
Vicat softening point than IPP with the above polymer may be used.
In such a case, the H layer should conveniently be made of a
constitution with a composition having better heat resistance than
the other layers. Heat resistance is a property possessed by the
resin itself or the mixed composition and a value expressed as the
synergistic effect with other layers during measurement in the use
as hereinafter described.
Next, referring to another layer of the invention, namely the
surface layer (S layer) having good sealing characteristic,
transparency and gloss, it comprises at least one polymer selected
from the above component (A), the component (B) and other
components as hereinafter described. They can be selected,
preferably from linear low density polyethylene (L-LDPE) as
described above, or ethylene-vinyl acetate copolymer (EVA),
ethylene-acrylic acid copolymer (EAA), ethylene-acrylic acid ester
copolymer (EEA), or a resin having at least a part of the product
saponified in at least a part of ethylene-acrylic acid ester and
ethylene-methacrylic acid ester ionically crosslinked (Io),
ethylene-.alpha.-olefin copolymer (preferably ethylene-butene-1
random thermoplastic elastomer), crystalline 1,2-polybutadiene,
etc.
In the case of L-LDPE, its preferable melt index is from 0.2 to 10,
and its density from 0.910 to 0.936 g/cm.sup.3, more preferably the
melt index is from 0.2 to 8 and the density from 0.910 to 0.925
g/cm.sup.3. Further preferably, the melt index is 0.2 to 6. The
lower limit of the melt index is due to the limit in extrudability
into a film, while the upper is due to instability in cold
stretching of the aforesaid main (base) layer brought about when
utilized as the surface layer, or insufficient seal strength at the
sealed portion or insufficient resulting strength. The lower limit
of density comes from the restriction in preparation of the resin,
while the upper limit is due to instability of stretchability
during drawing similarly as in the case of the upper limit of the
melt index as mentioned above and worsening of haze value or gloss
of the film, particularly after shrinkage. Within the above ranges,
without worsening workability and various characteristics or
inhibiting various characteristics of other layers, particularly
the base layer, but contrariwise through the synergistic effect
with other layers, these various factors were found to be markedly
improved. In particular, as to the characteristics, various
strength characteristics, sealing characteristic and high
temperature oil resistance characteristics can be markedly
improved. It is also preferred to use a polymer having a peak value
of 110.degree. to 125.degree. C. of the crystal melting point
(m.p.) as measured by the DSC method (temperature elevation speed:
10.degree. C./min.). The partner of the comonomer should preferably
be selected from octene-1, 4-methyl-1-penetene and hexene-1. The
linear low density polyethylene of the main component may also
contain other polymers mixed therein, provided that the various
characteristics as described above are not impaired thereby, with
the limit of other components mixed being 50 wt. %, preferably 40
wt. %, more preferably 30 wt. %.
Next, in the case of an ethylene-vinyl acetate copolymer (EVA), the
content of vinyl acetate is 3 to 10 mole %, preferably 3 to 7.5
mole %, more preferably 3.5 to 6 mole %, and its melt index
preferably 0.2 to 5, more preferably 0.5 to 3, further preferably
0.5 to 2.0. The same is the case with respect to other aliphatic
unsaturated monomer type copolymers. Also, in the case of the
ethylene-.alpha.-olefin copolymer, a preferable copolymer is an
ethylene-butene-1 random thermoplastic elastomer. In this case, MI
is 0.5 to 25. On the other hand, in the case of the crystalline
1,2-polybutadiene polymer, it is thermoplastic, has a crystallinity
of 10 to 35% and a melt flow index (150.degree. C.) of 1 to 10.
Among them, EVA is preferred and it is also possible to use other
kinds of resins mixed therein within the range which will not
impair the characteristics such as the sealing characteristic, haze
value or improvement of gloss, intended by the present layer,
particularly the characteristics exhibited through the synergistic
effect with other layers against worsening of haze value or gloss
asfter shrinkage. More specifically, its amount may be 50 wt. % or
less, preferably 40 wt. % or less, more preferably 30 wt. % or
less.
Either one or both of the skin layer and the base layer may employ
suitable additives (organic or inorganic) such as erucic acid
amide, oleic acid amide, stearic acid amide, bisamides of these and
others as the slipping agent, either singly or as mixtures. The
content thereof may range from 0.1 to 0.7 wt. %, more preferably
from 0.2 to 0.5 wt. %. Otherwise, singly or in addition to the
above additive, it is also preferred to use, as the anti-fogging
agent, polyalcoholic esters of fatty acids, nonionic surfactants of
olyoxyethylene alkyl phenyl ether and other effective ones. Typical
examples may include polyglycerine monoesters or diesters of oleic
acid, stearic acid or lauric acid, sorbitane monolaurate,
polyoxyethylene alkyl phenyl ether, monoleic acid glyceride, alkyl
alkylolamide, polyoxyethylene monooleate, polyoxyethylene
monostearate and the like. These can be used generally in an amount
of 0.3 to 5 wt. %.
It is also possible to add a small amount of a mineral oil as the
plasticizer to the above layer. Its preferred range is about 0.5 to
5 wt. %. Further, if necessary, as the tackfier (abbreviated as P
agent), for example, alicyclic saturted hydrocarbon resin, ester
gums, rosins, petroleum resins and terpene resins may be added to
the above layer. Its preferable range is 0.5 to 7 wt. %, more
preferably 1 to 5 wt. %. These various additives may be used as
desired depending on the purpose either singly or in a mixture, and
the total amount used may be approximately 0.5 to 15 wt. %,
preferably 1 to 10 wt. %.
The above additives, in order to exhibit further their effects, may
also be added not only in the skin layer but also in the aforesaid
base layer. In that case, in addition to the increased effect as
compared with the case where the respective additives are added
only in the S layer, bleed-out to the surface through the S layer,
the bleed-out speed can be controlled by the base layer as
described above, whereby there are additional effects such as the
initial effect of anti-fogging characteristic and synergistic
effect capable of exhibiting retentivity of the effect due to water
resistance with lapse of time. Also, as compared with the case of
employing these additives for a mono-layer film, for a reason not
clarified, staining phenomenon on the surface is particularly
small. This is one of the unexpected effects.
Further, by incorporating in at least the base layer of the above
layers, 1 to 30 wt. %, preferably 3 to 20 wt. %, more preferably 5
to 15 wt. % of at least one carrier (AS agent), which is selected
from alicyclic saturated hydrocarbon resins, ester gums, rosins,
petroleum resins, terpene resins, atactic polypropylene (APP),
1,2-PB, EVA with high VAc, oligomers or others and can retain the
above plasticizer and permit the anti-fogging agent to bleed out
gradually, a particularly excellent synergistic effect as mentioned
below can be exhibited. That is, the AS agent does not permit an
anti-fogging agent to bleed out within a short time, but
effectively retains by permitting it to bleed onto the surface
gradually, with the result that stickiness or staining on the
surface will hardly be generated. With a balance in amounts of
anti-fogging agent, plasticizer and AS agent, the antagonistic
requirements of plasticizing effect and nerve hardness can be
accomplished. This is very convenient for a film. It is surprising
enough that good anti-fogging property, slipping property and
wrapping property can be obtained in a thin film of about 5 to
10.mu.. As an additional effect, processability of the film,
particularly cold stretchability is improved and stabilized
(puncture is decreased, etc.). Further, a specific heat treatment
can act effectively, whereby elongating stress can readily be
lowered (namely readily denatured by partial deorientation, and
consequently readily elongated). During stretch wrapping, the
denatured portion is cold stretched again in the elongated
direction, and the film strength will easily be increased
non-linearly together with elongation (whereby wrapping becomes
easier). Such interesting effects can be exhibited by incorporation
of the AS agent. This effect may also be utilized for other layers,
particularly the skin layer, but in most cases sufficient effect
can be obtained by incorporation of the AS agent only in the base
layer.
With respect to the thicknesse of the respective layers, the
proportion of the base layer (M layer) within the total layer
thickness should preferably be 20 to 90%, more preferably 30 to
85%, further preferably 50 to 80%. The lower limit of the above
range is the proportion required in order to exhibit the
synergistic effect of the present invention by allowing the other
resin layer which cannot itself accomplish cold stretching to
accomplish the same stretching through the cold stretching force of
the base layer, and it is also the thickness necessary to exhibit
the various characteristics as mentioned above which are imparted
by the composition of the base layer. The ratio of the layer may be
determined optimally depending on the purpose of the film. For
example, when there is another layer with a composition to which
cold stretching can hardly be imparted, the lower limit of the base
layer thickness is relatively high. On the contrary, when there is
a compostion layer to which cold stretching can readily be
imparted, its level may of course be low, when considering only the
performance in processing. Actually, however, to have the
characteristics of the base layer other than processability (namely
cold stretchability) exhibited fully, the thickness may be
determined in view of the balance between both of these
characteristics. The upper limit in the above range is a ratio to
be determined by the effect of other layers utilized, and it can
freely be determined depending on the purpose.
The next layer, namely the core layer (H layer) as inside layer
comprising predominately the component (C) is higher in modulus
than other layers, and it is effective for improvement of the
modulus of the film as a whole as well as improvement of
dimensional stability and strength. Also, it is effective for
improvement of Vicat softening point, and heat resistance as
represented by the crystal melting point. All of these
characteristics can be exhibited for the first time when overlayed
on the base layer and subjected to cold stretching in the present
cold stretching method. Otherwise, if there is an appropriate resin
having a m.p. of 130.degree. C. or higher, it can also be used. The
content of the above component (C) may be 50 wt. % or higher,
preferably 60 wt. % or higher, more preferably 70 wt. % or higher.
Thus, the present film possesses at the same time the antagonistic
properties of the above tensile modulus, low temperature
shrinkability and stretchability during packaging. Moreover, the
film tensile modulus and dimensional stability are also improved as
compared with the above-mentioned prior Patent Application by the
present inventors. According to other methods than the present
invention, namely the high temperature stretching method, ordinary
low stretching method or unstretching method, no such layer as the
H layer as described above can be formed to accomplish no object of
the present invention, as a matter of course. The thickness of the
H layer relative to the total thickness may preferably be 3 to 40%,
more preferably 5 to 30%, further preferably 5 to 20%. The lower
limit is defined to have the above synergistic effect as the
present layer exhibited, while the upper limit is defined to give
the synergistic effect with the other layers, i.e., the base layer
and the skin layer as well as to give good processability. The
above ranges can conveniently be utilized for determining the layer
constitution depending on the purpose.
The next layer, namely the surface layer (S layer) may have a
proportion of thickness relative to the total layer thickness
preferably of 5 to 40%, more preferably of 10 to 40%, further
preferably of 15 to 30%. The absolute value of the total thickness
may preferably be 0.25 to 20.mu., more preferably 0.5 to 10.mu..
The lower limit of the above range is the minimum thickness
necessary to exhibit the effect of the S layer as mentioned above
synergistically with respective layers, while the upper limit is
necessary for processability and the characteristics of other
layers.
Examples of layer constitutions according to the respective
compositions as described above include the following combinations,
in which the base layer is represented by M, the core layer
comprising the composition (C) by H and the surface layer by S:
the case of three layers:
S/H/M
the case of four layers:
S/M/H/S, S/M/H/M, . . .
the case of 5 layers:
S/M/H/M/S, S/H/M/H/S, S/H/M/H/M . . .
the case of 6 or more layers:
S/M/H/M/H/S, S/M/H/M/H/M/S, S/H/M/H/M/H/S, . . .
In the above constitutions, the notation . . . /M/ . . . is not
limited to only one layer, but is also inclusive of two or more
layers of mixed resin layers with different compositions laminated,
for example, as indicated by . . . /(M).sub.1 /(M).sub.2 / . . .
.
Further, in addition to the above combinations, layers comprising
different kinds of resins such as of soft PVC may also be added by
appropriate means. Thus, the above examples are not limitative of
the invention.
The entire thickness of the film of the present invention may
generally be from 3 to 50.mu., preferably from 4 to 30.mu., more
preferably from 5 to 25.mu.. Particularly, for example, the film
for use in stretch wrapping for tray pack or non-tray pack is
required to have a very thin thickness of 4 to 20.mu., preferably 5
to 15.mu., more preferably 6 to 12.mu.. However, these ranges are
not limitative of the invention. The lower limit is thin enough due
to particularly higher strength of the film itself which enables
working with such a thin thickness, and itself can cope
sufficiently with other films because of having higher strength
than other films. With a thickness lower than the lower limit,
problems will ensue with respect to production and handling. The
upper limit can be defined, because the films with such a thickness
are sufficiently comparable to other thicker films in production as
well as characteristics exhibited. For example, the film of the
present invention with a thickness of 6 to 10.mu. is sufficiently
comparable to a soft PVC stretch film of about 18.mu. in thickness,
and yet surprisingly has a number of superiorities thereover.
The film of the present invention, in addition to bi-axial
stretching, can be stretched approximately mono-axially in either
the longitudinal or lateral direction depending on the manner in
which cold stetching is performed, but preferably stretched once
bi-axially, followed by a change in the degree of orientation in
either the longitudinal or transverse direction. This is a novel
aspect of the invention not found in the art, which lately or
simultaneously modifies the orientation (partial deorientation) to
impart elongation, whereby the film is strengthened by imparting
cold orientation again depending on the degree of stretching during
wrapping. As a result, in addition to the effect of easier
wrapping, this is one of the greatest points which can provide a
large number of specific features of the invention. These are the
results of a novel technique and method. To give additional
comments, the heat treatment method in general, for the purpose of
improving primarily dimensional stability, adopts the method of
heating the film after stretching under tension thereby to promote
crystallization and effect fixing, or aims to remove the component
of a small amount of strain (dimensional change generated with
lapse of time at room temperature) similarly under tension. In
contrast, the method of the present invention is different in
technology and object. Without recourse to the specific cold
stretching method of the present invention, it is difficult to
apply these treatments on the films obtained by the high
temperature stretching method as described above. Further, with the
use of a film according to the unstretched method, the film having
the characteristics of the invention can of course hardly be
accomplished. As described above, the present invention performs
preferably cold stretching at the initial stage. These facts
indicate that the present film is excellent also from the
standpoint of easier post-treatment.
The film of the present invention has a haze value (ASTM-D1003-52)
of 3.0% or lower, preferably 2.0% or lower. For example, in Run No.
1 of Example 1, it is very excellent as 0.3%. This is the value
characterized by the preparation method, and excellent transparency
can be obtained because working is possible without impairing the
properties of the composition quenched, namely the film can be
stretched stably in a bubble even at the temperature region lower
than the composition constituting the main component, more
preferably below its softening point (this cannot be applied, when
the film is colored or applied with printing). The low temperature
shrinkage as an additional characteristic of the present film other
than stretchability during packaging is at least 20% or more at
80.degree. C., and this characteristic can be utilized effectively
in the case of the product to be wrapped which cannot be wrapped
according to the stretch method alone. This is a matter which did
not happen in the film for stretch wrapping of the prior art.
More specifically, a square test strip cut from a film is marked
with lines of defined dimensions longitudinally and laterally, and
subjected to shrinkage by treating with hot air at a predetermined
temperature for 5 minutes with coverage of talc so that no sticking
to itself or other matters may occur. The changes in dimensions in
the respective directions after heat shrinkage are measured and
average values for both longitudinal and lateral directions
represent the percentage of heat shrinkage.
The tensile modulus of the film in the present invention is
specific in that it can be controlled freely from relatively low
tensile modulus to relatively high tensile modulus by changing the
constitution of the specific layer (base layer) or the constitution
of the H layer or the thicknesses of both, compositions, etc.
Further, the film of the present invention has a particularly
strong tensile strength, having a tensile strength at break of at
least 4 Kg/mm.sup.2 (value measured according to the method of ASTM
D882-67), preferably 5 Kg/mm.sup.2 or more, more preferably 6
Kg/mm.sup.2 or more, in its stronger direction, preferably as mean
value in both directions, more preferably in both directions, with
its tensile elongation being at least 150% or higher, preferably
200% or more, more preferably 250% or more, in the direction
elongatable longer, while at least 100% or more, preferably 150% or
more, more preferably 200% or more in the direction perpendicular
thereto.
The fact that the film has such a strong tensile strength and large
elongation means that the film is tough and difficultly broken,
which is very advantageous as the protective film for packages.
Thus the film thickness can be saved.
The film of the present invention can have the levels of strength
at break (average of longitudinal and lateral directions) of 7.1
Kg/mm.sup.2 and elongation of 310% as hereinafter described (Run
No. 1). Usually, when the strength is increased by orientation
according to the prior art method as described above, elongation
tends to be lowered extremely. For example, the film (Control b
hereinafter described) of a single material of low density
polyethylene which has been sufficiently crosslinked (67 wt. % of
insoluble gel in boiling xylene) and sufficiently oriented at high
temperature has a strength of 6.9 Kg/mm.sup.2, with the elongation
being very low as 110%, and the sress relative to elongation is too
high. As a result, such a film will readily be broken, hardly
elongated and therefore stretch wrapping is entirely impossible
with such a film.
Stretchability is the most important factor when stretch wrapping
by a machine or particularly with hands. First of all, the stress
at 100% elongation should be 100 to 600 g/cm-width, preferably 150
to 500 g/cm-width, more preferably 200 to 400 g/cm-width as a
practical value represented in terms of a mean value of the
longitudinal and transverse direction values. The preferable
balance of longitudinal/transverse direction in case of wrapping by
drawing transversely is 5/1 to 1/1, more preferably 5/1 to 4/3.
Secondly, the stress on 200% stretching should preferably be 200 to
1000 g/cm-width, more preferably 250 to 900 g/cm-width, more
preferably 300 to 600 g/cm-width. In the case of wrapping by
drawing the film in the transverse direction, the balance ratio of
longitudinal/transverse stress should preferably be 5/1 to 1/1,
more preferably 5/1 to 4/3. The value in the transverse direction
should be 100 to 400 g/cm-width, preferably 150 to 350 g/cm-width,
more preferably 200 to 300 g/cm-width. (During measurement, when
the film is broken before reaching a certain value due to
insufficient elongation, the value is determined by
extrapolation).
As viewed from orientation characteristic, the stress on an average
(of longitudinal and transverse) on 100% elongation is 1 to 6
Kg/mm.sup.2, preferably 1.5 to 5.0 Kg/mm.sup.2, more preferably 2
to 4.0 Kg/mm.sup.2. Similarly, the stress on 200% elongation is 2
to 10 Kg/mm.sup.2, more preferably 2.5 to 8.0 Kg/mm.sup.2, still
more preferably 3.0 to 6.0 Kg/mm.sup.2, and the balance is on the
same level as mentioned above.
In other words, since the film according to the present method has
a specific layer constitution in which cold stretch is imparted in
at least one direction to all the layers, by stretching the film
loosely wrapped first on an article to be wrapped during wrapping
in the direction with greater elongation, the stress can also
extend to the direction perpendicular to the direction to which the
stress is applied, whereby the loose portion can be finished
tightly and further the orientation of the film is changed to the
direction to which stress was applied to result in further
improvement of the film. Thus, it has been found that cold
stretching orientation is imparted again to the film. This fact
reflects the specific feature of the film according to the cold
stretch method as compared with the films of other methods, which
is high in orientation degree and also great in residual elongation
at the same time. As a result, as compared with the prior art, it
has been found that stress can be propagated in respective
directions, and the film in the direction loosened can be migrated
to readily effect packaging of articles with good finishing.
The film of the present invention, due to its specific layer
construction, is excellent in handling performance, and also
excellent in heat resistance, particularly hole opening resistance
by melting at the seal portion during sealing between film
surfaces, with the result that the sealing range is broad. Further,
for example, when there exist a portion with several films
overlapped and a portion of one film at seal portion at the bottom
of a tray, an exceptional performance can be exhibited (in respect
of satisfying the antagonistic properties of heat resistance and
sealability). Also, the present invention was successful for the
first time in making the film thickness practically very thin,
while maintaining its strength characteristic and handling
characteristic. For example, by the test in the market, wrapping
was found to be possible by use of the film of the present
invention with a very thin thickness of 10.mu., in place of a
plasticized PVC stretch film with a thickness of 18.mu., while
satisfying the respective requirements. An ultra-thin film of 7.mu.
is also available. It has also been found that an article which has
been wrapped with a plasticized PVC stretch film with a thickness
of 26.mu. due to stringent requirements for characteristics can be
wrapped with a film of the present invention having a thickness of
10.mu..
In addition to the specific features as described above, it is also
possible to take the advantage of the specific feature of low
temperature shrinkability. This is a particularly desirable
property in the case of using a tray of not square form or when the
article to be wrapped cannot be well finished only by stretch
wrapping. During sealing, by the heat at the sealed portion, the
portion near the sealed portion or whole of the film is shrunk
simultaneously with sealing, whereby wrapping can be finished
tightly. Further, even in the case of finishing according to shrink
wrapping without stretch, complete wrapping can be accomplished
rapidly by use of the film of the present invention under more
advantageous conditions, such as hot air with lower temperature
than the shrink wrapping method of the prior art. Improvements are
also realized with respect to the extent of maintenance of heat at
the sealed portion by covering over the sealed portion, very simple
method of applying only stirring of the air, and better heat
efficiency without causing denaturation of the article to be
wrapped by giving heat thereto. Wrapping by way of
stretch-and-shrink wrapping with the use of such a film as
described above has never been accomplished in the art, and it can
be another effective use and specific feature of the present
invention in practical application, whereby the scope of wrapping
can be widened to a great extent.
Having described the present invention by referring to the use as
described above, the present invention is not limited only to such
a use, but it is an epoch-making film which can be utilized for
various kinds of uses.
The film of the present invention is a film endowed with
antagonostic properties of heat resistance, shrinkability and
sealability with good balance, and its specific improvement resides
in that the value of heat resisance [T.sub.H ] .degree. C. and the
value of seal temperature [T.sub.S ], as measured by the methods in
Examples as hereinafter described, satisfy the relationship in
terms of the value of [T.sub.H -T.sub.S ], which is at least
15.degree. C., preferably at least 25.degree. C., more preferably
at least 35.degree. C.
The present invention is described by referring to the preferred
embodiment of the present invention, by which the present invention
is not limited at all.
According to the process of the present invention, by means of
respective extruders, the compositions for respective layers are
thermoplastified to be molten so that the polymer compositions and
the respective layer constitutions may be obtained, extruded
according to the method in which the molten resins are extruded
through a multi-layer die, the method in which the molten resins
are made confluent before extruded through a die or the method in
which a resin film extruded through a die is succesively coated
with other resins, and thereafter quenched with a liquid coolant to
be solidified, thus forming a sufficiently uniform tubular or sheet
raw film. In this case, it is preferred to extrude the resins
through an annular multilayer die to form a tubular raw film,
although the present invention is not particularly limited
thereby.
The resultant raw film obtained having the respective layers as
constituent layers may be pre-treated to the extent as described
above with a high energy ray, such as electron beam, gamma-ray,
UV-ray, to a dosage of, in the case of electrom beam, 1 to 10
Megarad, preferably 3 to 7 Megarad. Excessive treatment is not
desirable, because unfavorable results may be brought about to
various characteristics.
As the next step, the raw film, heated to 100.degree. C. or lower
or as such, is subjected to cold stretching at a stretching
temperature of 30.degree. C. to 80.degree. C. to an area stretching
ratio of 4-times or more to 30-times. The stretching temperature as
herein mentioned represents the temperature on initiation of
stretching.
Then, the stretched film is specifically modified by changing the
orientation by permitting shrinkage of the film at 40.degree. to
100.degree. C. to at least 5 to 50% (area), preferably at
45.degree. to 80.degree. C., more preferably 45.degree. to
70.degree. C., with the degree of shrinkage being preferably 5 to
40 (area) %, more preferably 7 to 30 (area) %.
In the following, the present invention is described by referring
to a preferred embodiment, by which the present invention is not
limited.
Heating of the raw film may be conducted at 100.degree. C. or
lower, preferably 90.degree. C. or lower, more preferably
85.degree. C. or lower. More preferably, a desired film can be
obtained conveniently for the first time by heating the film to a
temperature without melting the crystal components of the resins
constituting mainly the base layer (M layer), the surface layer (S
layer) and the H layer and without impairing the properties after
quenching, and expanding the film in a bubble under sufficient
inner pressure at a temperature lower than the melting points of
the original crystal components constituting the main components of
the respective compositions of the above layers of 80.degree. C. or
lower, preferably 35.degree. to 70.degree. C., more preferably
35.degree. to 65.degree. C., more preferably at a temperature not
higher than the Vicat softening point of the original polymer or
mixture. The optimum degree of area stretching ratio depends on the
respective compositions, layer constitutions and the temperature
employed, but is generally 4 to 30-times, preferably 5 to 20-times,
of which the degree of stretching in the lateral direction in the
preferable case of bi-axial stretching is generally 2 to 6-times,
preferably 2 to 4-times. As the conditions for preventing puncture
in stretching and affording sufficient cold stretching, the
respective compositions and layer combinations within the ranges as
specified above are particularly important and at the same time it
is also important to prepare a sufficiently uniform raw film.
Stretching can be practiced most stably while regulating expansion
of the film in the lateral direction by deflating immediately at
the maximum diameter portion by means of a roll type deflator.
Also, the raw film bubble is conveniently of a large size as
permissible by the device, for example, about 30 mm or more in
diameter, preferably 50 mm or more in diameter, in view of the
relationship between the inner pressure and the diameter. In view
of the physical properties of the film obtained, stretching should
be done under sufficiently cold conditions, so far as permitted by
stability of the bubble. Practically, however, the extent of
stretching may be determined according to the composition employed
in view of the balance with stability (so as to cause no
puncture).
The cold stretching can be stable and the entire thickness of the
film can be made uniform, because the stretching temperature is
lower than that of conventional process and further there is the
synergistic effect of stretching to high degree of the respective
layers of the multilayer, whereby the all of layers can be
stretched to high degree to give a film having the characteristics
as described above. Also, the film thickness can be controlled
freely from very thin to thick.
In contrast, according to the conventional stretching method by
heating to a temperature higher than the melting method, the
stretching temperature must be elevated contrariwise to higher
temperature in order to improve optical characteristics, whereby
the film will become hardly oriented and the strength of the film
tends to be lowered in most cases.
The situation is the same at a temperature of from .+-.5.degree. to
10.degree. C. of the melting point of the main component of the
polymer. Further, not only can no preferable result be obtained
with respect to haze or gloss, but also the raw film of the mixed
composition becomes brittle at the temperature conditions employed
to be sometimes punctured, whereby high characteristics can hardly
be imparted to the film. According to a conventional method for
combination of layers, when different kinds of resins are combined
in a multilayer, the resins have respective optimum stretching
temperatures which differ greatly from each other. As a result,
most of the combinations cannot be oriented as a whole. Thus,
usually, it frequently occurs that only a part of the layers can be
subjected to orientation by stretching at the sacrifice of other
layers.
As described in Examples hereinafter, stretching of all of the
layers as mentioned in the present invention can be accomplished
successfully. No such stretching has been known in the art. It can
be accomplished through the synergistic effect by use of, for
example, a multilayer tube containing a specific mixed composition
layer, a uniformly quenched raw film and the conditions of the
specific stretching method.
The heating temperature as herein mentioned refers to the maximum
temperature in the raw film before stretching, and the stretching
temperature mentioned in the present invention refers to the
temperature at the portion at which stretching is initiated,
wherefrom, as a matter of course, the temperature is lowered by
cooling to the region where stretching is completed, as a matter of
course. At the region where stretching is completed (the region
where the bubble reaches its maximum diameter) or thereafter, the
film is cooled sufficiently at least to 45.degree. C. or lower,
preferably 35.degree. C. or lower, more preferably 30.degree. C. or
lower. Thus, it is preferred to control the temperature difference
between the stretching initiation portion and the completion
portion at 5.degree. C. or higher, preferably 10.degree. C. or
higher, more preferably 15.degree. C. or higher. These values are
given when temperatures are measured by means of contact type
thermometer from the surface. For example, in the case of Run No. 9
in Examples, the temperature at the stretching initiation portion
was found to be 53.degree. C., while those between the portion at
the maximum bubble diameter and the raw film during expansion were
50.degree. C. at 1/3 length from the raw film, 39.degree. C. at 2/3
length from the raw film and 25.degree. C. at the completed region,
respectively. As described above, the process of the present
invention can be understood to be a cold stretching method not
found in the prior art.
The stretching, in order to carry out high degree of cold
stretching smoothly, should preferably be conducted while
controlling the air stream at the surface layer portion as
uniformly as possible by effecting uniformly heating and blowing
air conditioned through air-ring, etc. The raw film should
preferably be heated at a temperature not exceeding by 20.degree.
C. the temperature at the portion where stretching is initiated, in
view of the stability at the start-up.
As a method for controlling the air stream at the surface layer
portion, there is, for example, the method in which a rectifying
contact guide intended for separating substantially the heating
portion from the stretching initiating portion is used, and the
fluid (air) accompanying the film surface and its boundary film are
removed incontinuously by contact thereby removing nonuniformity
caused by the interaction between the heating portion, the
stretching initiating portion and the cooling portion. This method
can also be employed similarly at the stretching initiating
portion, the stretching portion and the stretching completion
region. It is peferred that the stretching should be sufficiently
stretched in a higher inner pressure within the bubble, for
example, a high pressure of from 100 to 5000 mm of water column
(H.sub.2 O) (on the raw film base of 200.mu. in thickness and 100
mm in diameter), more preferably from 200 to 2000 mm (H.sub.2
O).
The film obtained according to the process of the present invention
has the excellent physical properties as described above, and at
the same time has little thickness nonuniformity after stretching
in most cases. This may be considered to be due to good uniformity
and stability obtained because of the fact that stronger elongating
force is imparted to the film by the high inner pressure in the
bubble and also that heat hysterisis by heating and cooling is
particularly small. As to haze and gloss, while they may appear
more or less poorer at the stage of raw film, they can be improved
very well after the cold stretching according to the present
invention, as one of the characteristics of the invention. Also, by
making the multilayer as described above, stability during working
can be improved to a great extent to give articles of uniformly
high grade, as compared with the case of single layer.
For example, in the case of a single layer of polypropylene, it can
be stretched only under a very narrow range, and stretching can
hardly be conducted. Continuous stretching can be accomplished only
under severe conditions and no stretching is possible under the
conditions lower than said range with accompaniment of puncture or
only a weak and inferior film whitened can be obtained under the
conditions higher than said range. Also, at a temperature around
80.degree. C., or still harder as in the case of the above example,
at 35.degree. C., for example, stretching cannot be accomplished at
all. This is an entirely surprising fact.
The characteristics obtained of the multilayer film of the
invention are more excellent in strength, haze, gloss, low
temperature shrinkability, sealability, tearing strength and impact
strength as compared with those of a single layer, which are higher
in level than the film obtained according to conventional
stretching. Next, the film of the present invention is also
characterized by that, after stretching, it is subjected to a
post-treatment on line or after wind-up, as desired, to effect a
part of deorientation of the oriented film to impart elongation and
orientation to the film by packaging, and the extents of
orientations in the longitudinal and transverse directions may be
changed as desired. As a preferable method, the longitudinal and
transverse orientations of the film may be modified through the
effect of shrinkage between nip rolls to an area ratio of 5 to 50%,
preferably 5 to 40%, more preferably 7 to 30% against original
dimension before post-treatment depending on the desired
characteristics and also the effect of application of heat itself.
The temperature for the application of heat may be from 40.degree.
to 90.degree. C., preferably from 45.degree. to 80.degree. C., more
preferably from 45.degree. to 70.degree. C. Within the conditions
as specified above, the above treatment is required to be conducted
while maintaining the excellent properties obtained by cold
stretching, for example, in order to further impart elongatability
to the film, while maintaining its strength, and such a treatment
is preferably conducted at a temperature not higher than the m.p.
of the surface layer, more preferably also at a temperature not
higher than the m.p. of the polymer which is the main component of
the base layer.
In preparation of the film, the so called edge portions at both
sides of the film may sometimes be cut off. Such edge portions also
comprise at least three layers of S layer, M layer and H layer, but
the edge portions of the present film can be utilized again as the
starting material for M layer. It is preferred that the content of
the edge portions for reutilization charged into the extruder for M
layer should be 30 wt. % or less. Thus, the relatively expensive
starting material to be used in the present invention can be
reutilized without being discarded to result in cost reduction.
This is another advantage of the present film.
EXAMPLE 1
A resin mixture (M.sub.111-3) (VSP: 78.degree. C.) was prepared by
mixing 65 wt.% of a.sub.1 : an ethylene-vinyl acetate copolymer
[vinyl acetate content: 3.5 mole %, MI: 1.0, m.p.: 95.degree. C.,
VSP: 79.degree. C.], 20 wt. % of b.sub.1 : an
ethylene-.alpha.-olefin copolymer thermoplastic elastomer
[.alpha.-olefin is propylene: 15 mole %, 3 wt. % of
ethylidenenorbornene randomly copolymerized, MI: 0.45, VSP:
40.degree. C. or lower, density: 0.88 g/cm.sup.3 ], 10 wt.% of
c.sub.1 : a crystalline polypropylene (IPP) [randomly copolymerized
with 4 wt. % of ethylene, MFR: 7, m.p. 143.degree. C., VSP:
135.degree. C., density: 0.90 g/cm.sup.3 ] and 5 wt. % of C.sub.3 :
a crystalline polybutene-1 (PB-1) [MI: 1.0, density: 0.905
g/cm.sup.3, butene-1 content: 96 mole %, VSP: 103.degree. C., m.p.
117.degree. C.]. A mixture (M.sub.111-31) for the base layer was
prepared by kneading of 100 parts by weight of the resultant
mixture (M.sub.111-3) with 10 parts by weight of an alicyclic
saturated hydrocarbon resin (AS.sub.1) as the AS agent. Next, as
the resin for H layer, 80 wt. % of c.sub.1 : IPP (as described
above) and 20 wt. % of c.sub.3 PB-1 (as described above) were mixed
together to a composition (H.sub.1). Also, as the resin for S
layer, a.sub.2 : EVA [vinyl acetate content: 5.5 mole %, MI: 1.0,
m.p. 88.degree. C., VSP: 74.degree. C.] was employed. The above
resins, in the order mentioned, were thermoplastified in three
extruders, respectively, a first one having a screw of 50 mm
diameter (L/D=37) and an injection orifice at the position of L/D=8
from the end portion, a second one having a screw of 40 mm diameter
(L/D=29) and a third one having a screw of 40 mm diameter (L/D=37)
and an injection orifice at the position of L/D=8 from the end
portion, at the maximum temperature of 220.degree. C. at the
cylinder portion. In carrying out extrusion, two kinds of
anti-fogging agents of diglycerine monooleate and polyoxyethylene
nonyl phenyl ether and additives such as mineral oil as the
plasticizer were injected to those layers finally in amounts of 1.5
wt.%, 1.0 wt. % and 0.5 wt. %, respectively, 3.0 wt. % as the total
(in Run No. 4 and No. 5, the total amount was made 1.5 wt. %)
through the injection orifices at the extruders for the base layer
and S layer. These compositions were kneaded and co-extruded
through a five-layered annular die. Then, the extruded product was
quenched at the position which was about 6 cm distance from the lip
end of the die by the water-cooling ring from which water was
uniformly flowed out to obtain various raw films with a diameter of
180 mm. They are shown in Table 1. In Run No. 4 and No.5, only
PB-1:c.sub.3 was employed for H layer.
TABLE 1 ______________________________________ Run No. Control 1 2
3 4 5 1 2 3 ______________________________________ Layer struc-
ture of raw film First(S) 5 7 11 20 30 -- -- -- layer .mu.
Second(M) 15 25 33 60 90 -- -- -- layer .mu. Third(H) 10 7 22 40 60
120 -- -- layer .mu. Fourth(M) 15 24 33 60 90 -- 120 -- layer .mu.
Fifth(S) 5 7 11 20 30 -- -- 120 layer .mu. Total 50 70 110 200 300
120 120 120 thickness (.mu.)
______________________________________
In each raw film, thickness variation (circumferential direction)
was less than .+-.2%. These raw films were passed through two pairs
of delivery nip rolls and draw nip rolls and heated therebetween to
45.degree. C. with hot air, followed by inflation with air as such
at a stretching temperature of 44.degree. C., and expanded
continuously by means of a rectifying contact guide to be stretched
approximately to from 3.3 to 3.6 times in the longitudinal
direction and from 3.2 to 3.4 times in the transverse direction.
The end region of stretching is cooled with an air ring through
which cold air of 18.degree. C. is blasted, and the stretched film
was folded with a deflator, drawn up with nip rolles, air of
50.degree. C. was blown against the film, and the film was further
subjected to shrinking in the heat treatment zone between to pairs
of nip rolles slower by 15% in the outlet speed to predetermined
rates of 15% in longitudinal direction and 10% in lateral
direction, simultaneously with application of stabilizing
treatment. The film was separated into two sheets of films by
slitting its edge and wound up under respective tensions to obtain
the desired films with respective thicknesses. Table 2 shows the
various characteristic values of the films obtained and four kinds
of commercially available films a, b, c and d which are
Controls.
The raw film of Control 1 could not be stretched under the same
conditions but punctured, and it was gradually elevated in
temperature up to 150.degree. C. At a temperture of 140.degree. C.,
it could be at last stretched. This film had a white appearance,
with a stretching characteristic prone to breaking and without low
temperature shrinkability, namely having a shrinkage of 2 to 3% at
80.degree. C.
On the other hand, the sample of Control 2, could be stretched
under the same conditions, but lacking slightly stability as
compared with those of Run No. 1-5, being also deficient in
anti-fogging characteristic with staining on the surface. Further,
the sample of Control 3 was unstable in stretching and is readily
punctured. The film of Control 2 had excellent gloss and haze value
as such, but worsened to a haze value of 8% when shrunk
sufficiently to 40%. The film of Control film 3 is excellent in
both in gloss and haze value, but instable in film dimensions at
room temperature and devoid of heat resistance, being susceptible
to melting with formation of holes when sealing of the film is
attempted.
Other samples of Run No. 1-5 were stable in stretching without
staining on the surface, and also excellent in anti-fogging
characteristic.
TABLE 2 Run No. Control Control Control Control Control Control
Character istics Unit 1 2 3 4 5 2 3 .circle.a .circle.b .circle.c
.circle.d Haze % 0.3 0.5 0.7 1.0 1.1 1.7 0.5 1.7 2.5 2.0 2.0 Shrink
percentage % 30 35 36 37 34 40 57 33 5 0 0 (80.degree. C.) Tensile
strength Kg/mm.sup.2 7.1 8.2 9.1 7.6 7.0 6.5 4.9 8.0 6.9 2.2 3.1
Tensile elongatio n % 250/370 260/320 290/350 280/360 250/350
220/280 200/280 120/170 100/120 200/400 250/300 (LD/TD) Stress on
100% Kg/cm 290/120 300/150 400/190 450/160 600/290 280/130 270/100
500/1400 1100/1200 200/120 350/150 elongation (LD/TD) width Stress
on 200% Kg/cm 550/200 600/230 650/280 800/350 1200/450 500/250
500/230 --/-- --/-- 250/140 500/250 elongation (LD/TD) width Dart
impact Kg .multidot. cm 14 18 24 36 49 25 16 16 8 6 7 strength *1
Sealing .degree. C. 85 87 88 95 110 128 90 160 170 100 180
temperature *2 Heat resistant .degree.C. 150 150 155 160 170 125 87
150 165 95 165 temperature *3 Hand wrappability *4 --
.circleincircle. .circleincircle. .circleincircle. -- -- x x x x
.DELTA. .circle. Anti-fogging -- .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .DELTA.
.circleincircl e. .DELTA. .DELTA. .circle. .circle. property *5
Surface staining -- .circleincircle. .circleincircle.
.circleincircle. .circle. .circle. .DELTA. .DELTA. .circleincircle.
.circleincircle. .circleincircle. .circle. resistance *6
Sealability *7 -- .circleincircle. .circleincircle .
.circleincircle. .circleincircle. .circle. .DELTA. x .DELTA.
.DELTA. x x Film thickness .mu. 6 8 13 25 38 14 15 13.5 15 21
*LD/TD = longitudinal direction/transverse direction. *1 In
carrying out measurement according to ASTMD-1709-67, a head was
employed in which a grooveedge portion was provided in the missile
head t effect bursting more easily. *2 The lower limit of the
temperature, at which, when a tray of a commercially available high
impact Styrol (10 cm .times. 20 cm) is wrappe with a film to form a
portion with folding of four films, a portion with folding of two
films and a portion with one film and the films with two o more
films are sealed by pressing under a force of 2 g/cm.sup.2 for 3
sec., and not peeloff occurs even when the end portion may be
lightly drawn. *3 The temperature, at which a hole is formed
somewhere in the film, when the folded portion of the films is
pressed under a pressure of 4 g/cm.sup.2 for 3 sec. according to
the above method *2. (provided that, when the film thickness was
20.mu. or more, evaluation was conducted for the portion with two
films laminated, both in cases of *3 and *4). *4 The ranks of
evaluation of wrapping without generation of creases, wit good
stretchability, without breaking and with neatness, when hand
wrapping is conducted with two oranges places on a tray made of an
expanded polystyrene by means of a commer cially available hand
wrapper for stretch wrapping: .circleincircle.: all could be
wrapped best; .circle. : wrapped considerably well; .DELTA.:
creases remained partially, with the film being easily broken; x:
unable to remove creases at all, film broken or tray broken, thus
enabling no wrapping at all. *5 Film was extended over a vessel
holding water of about 15.degree. C., and the water droplets
attached there on, when left to stand in a refrigerator of
5.degree. C. for ten minutes, were observed: .circleincircle.:
uniform water layer formed to enable complete observation of the
inner portion therethrough; .circle. : water layer not completely
formed, with uneveness, thus affording observation only of the
inner portion in deformed state; .DELTA.: water droplets attached
with large droplets, with only a part of the inner portion being
seen; x: water droplets attached wholly to permit no observation of
the inner portion. *6 Bleeding irregularity of the additive on the
film surface was examined .circleincircle.: entirely uniform;
.circle. : slightly thin pattern generated; .DELTA.: pattern
generated all over the surface; x: the surface stained and sticky.
*7 The level of sealability with the hot plate at the bottom
portion during wrapping in the above *4 was examined by selecting
the optimum temperature: .circleincircle.: sealing effected
substantially completely over the entire surface with broad
temperature conditions (i.e. difference between the sealing
temperature in *2 and heat resistant temperature in *3 is great);
.circle. : 60-80% of the entire surface sealed; .DELTA.: spot
sealing is possible, but holes formed when attempted to sea the
entire surface; x: a part sealed, but simultaneously with melting
of other portions to form holes.
Control sample of (a) is a commerically available PVC shrink film
moderately plasticized (20 wt. %.)--oriented film--.
Control sample of (b) is a commerically available crosslinked
polyethylene shrink film--oriented film--.
Control sample of (c) is a commerically available non-PVC type (EVA
containing 20 wt. % of VAC) stretch film--non-oriented film--.
Control sample of (d) is a commerically available highly
plasticized (31 wt. % of plasticizer) PVC type stretch
film--non-oriented film--.
The films obtained were found to be excellent in all the
characteristics and more excellent than the samples of
Controls.
The films of Run No. 1 and No. 2 could be wrapped excellently with
sufficiently good finishing and sealing with the hot plate, while
clearing the problems in the respective wrapping steps, by means by
any kind of wrapping machines. For example, commercially available
automatic wrapping machines for stretching wrapping may be
classified into those of the pillow system wherein the film is
stretched along the flow direction in the width direction and the
film edges are folded in both ends of the tray and those of the
type in which the film is cut to desired sizes according to the
thrusting system and drawn from the four directions, with the
product carried on the tray being thrusted atainst the film and the
film being folded from the four directions. Thus, the films of the
present invention are available in either type of these wrapping
machines. Also, when a cover is placed on the hot plate for sealing
to maintain the heat in the sealing portion, or a hot air of
70.degree. C. or lower is circulated, a wrapped product in the form
raised above the tray can be completely finished in wrapping, even
when loosely wrapped. Such a wrapping form (stretch+shrink
wrapping) is one of the wrapping methods, taking sufficient
advantage of the specific feature of this film.
Also, good wrapped products could be obtained, when the films of
Run No. 1, 2 and 3 were used for wrapping by means of the
hand-wrapper wrapping machine (manual machine) with stretching.
Next, as the practical shrink wrapping test, with the use of the
films of Run No. 1, 2 and 3, four cucumbers were wrapped loosely by
means of a commercially available L-type wrapping machine, with the
end portions being sealed by melting to be formed into bags,
followed by passing through a commerically available tunnel with
blowing of hot air of 90.degree. C. for two seconds, whereby shrink
wrapping could be effected tightly without any crease with
excellent fitting and wrapped finishing, thus giving beautiful
shrink wrapping without worsening of optical characteristics after
shrinkage. Also, good wrapping results could be obtained within a
broad temperature range from lower temperature side and within a
broad speed range. Such a shrink wrapping method is also one of the
wrapping methods which can make use of the specific features of the
present invention. It was also confirmed that a loosely wrapped
tray pack could be tightly wrapped only by treatment with hot air
at 70.degree. to 80.degree. C. for 0.5 to 1 sec. No such result was
possible with the use of the films of the Controls (a), (b), (c)
and (d).
As described above, the film of the present invention has more
specific features in uses of stretch films than the products of the
prior art and can completely satisfy these uses. At the same time,
it is also available in uses for shrink wrapping of the prior art
or novel and more advantageous wrapping method as the film for
stretch-shrink wrapping in which the advantages of the both
wrapping methods have been incorporated. In particular, the film of
the present invention is suitable as the film of stretch wrapping,
and it can sufficiently compete with a plasticized PVC type film
with a thickness of 18.mu. and an EVA type stretch film with a
thickness of 20 to 22.mu., even with a thickness of 6 to 8.mu., and
it is also a multi-purpose film having epoch-making performance and
expected to contribute much to the world.
EXAMPLE 2
According to the same procedure as described in Example 1,
employing further one additional extruder as described above having
a screw with 40 mm diameter (L/D=37), the raw films having the
respective compositions and the combinations of layers as shown in
Table 3 were obtained according to the method of adding the
additives as hereinafter described. Among these, in Run No. 14, the
raw film was irradiated with an electron beam (energy of 500 kV) as
the energy ray at a does of 5 Mrad (insoluble gel of the base
layer: 12 %, MI of the base layer: 0.08). Then, similarly as
described above, the heating temperature was suitably selected
within the range from 40.degree. to 60.degree. C., and cold
stretching was performed at stretching temperatures of 35.degree.,
47.degree., 45.degree., 53.degree., 46.degree., 45.degree.,
44.degree., 40.degree., 46.degree. and 45.degree. C., respectively,
for Run No. 6-14 and Control 4 (these temperatures are all lower
than the Vicat softening points of the polymer mixtures for the
base layers as described below), followed by the post-treatment as
described in Example 1 to obtain the films as shown in Table 4.
In the above Run No. 7, 8, 9, 12, 13, and 14, the carrier resin of
an alicyclic saturated hydrocarbon resin (AS-1) was added to 100
parts of the respective resins for the base layers in amounts of 5,
3, 15, 7, 10 and 7 parts by weight and kneaded. Also, as the
anti-fogging agent, a mixture of oleic acid monoglyceride and
polyglycerine monolaurate at a weight ratio of 1:2 was added in
each layer at a proportion of 2.5 wt. % and kneaded, followed by
extrusion and stretching as described above.
TABLE 3
__________________________________________________________________________
Layer Run No. structure Control of raw film 6 7 8 9 10 11 12 13 14
4
__________________________________________________________________________
First layer .mu. a.sub.1 a.sub.1 a.sub.1 a.sub.2 a.sub.1 a.sub.2
a.sub.2 a.sub.2 a.sub.10 a.sub.2 (S layer) 11 6 7 18 10 12 10 8 11
18 Second layer .mu. M.sub.112 M.sub.321 M.sub.511-3 M.sub.1-611
M.sub.131 M.sub.141 M.sub.151 M.sub.111-3 M.sub.111-3 M.sub.112
(Base layer) 28 15 15 24 20 22 30 20 30 84 Third layer .mu. H.sub.2
H.sub.1 H.sub.0 H.sub.3 H.sub.4 H.sub.1 H.sub.3 H.sub.4 H.sub.1 --
(H layer) 33 18 20 36 16 12 5 21 11 Fourth layer .mu. M.sub.112
M.sub.411 M.sub.511-3 M.sub.1-611 M.sub.131 M.sub.111-3 M.sub. 110
M.sub.011 M.sub.1-611 -- (Base layer) 28 15 25 24 26 22 27 14 47
Fifth layer .mu. a.sub.2 a.sub.1 a.sub.1 a.sub.2 a.sub.1 a.sub.2
a.sub.2 a.sub.2 a.sub.10 a.sub.1 (S layer) 11 6 8 18 8 12 8 7 11 18
Total (.mu.) 111 60 75 120 80 80 80 70 110 120 thickness
__________________________________________________________________________
TABLE 4
__________________________________________________________________________
Run No. Control Characteristics Unit 6 7 8 9 10 11 12 13 14 4
__________________________________________________________________________
Haze % 0.4 0.3 0.5 1.0 0.8 0.6 0.7 0.9 0.6 0.9 Shrink percent- % 31
30 34 30 31 30 28 35 37 35 age (80.degree. C.) Tensile strength
Kg/mm.sup.2 6.9 7.5 8.3 7.0 7.5 8.1 7.2 6.7 10.0 6.0 Tensile %
270/350 250/370 240/330 290/350 310/310 240/290 230/300 220/360
210/340 200/250 elongation (LD/TD) Stress on 100% Kg/cm 290/180
310/150 270/140 350/195 280/170 260/210 250/180 245/190 330/210
300/200 elongation width (LD/TD) Stress on 200% Kg/cm 600/280
550/290 590/250 610/380 570/300 580/320 560/250 530/220 650/320
700/355 elongation width (LD/TD) Sealing temper- .degree.C. 84 87
86 88 90 86 87 88 92 85 ature Heat resistant .degree.C. 140 155 150
147 153 150 141 144 175 110 temperature Sealability --
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .DELTA. Hand -- .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circle.
wrappability Anti-fogging -- .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
1 property Surface staining -- .circleincircle. .circleincircle.
.circle. .circle. .circleincircle. .circle. .circleincircle.
.circleincircle. .circleincircle. .circle. resistance Film
thickness .mu. 12 7 8.5 15 9 10 9 8 13 15
__________________________________________________________________________
The respective symbols in the Table show the following resins or
resinous compositions:
M.sub.112 : A mixture comprising 65 wt. % of EVA (a.sub.1), 15 wt.
% of the elastomer (b.sub.1) and 20 wt. % of an IPP [MFR: 2.2,
m.p.: 164.degree. C., VSP: 151.degree. C., density: 0.91 g/cm.sup.3
] (c.sub.2), said mixture having a VSP of 76.degree. C.;
M.sub.321 : A mixture comprising 60 wt. % of EVA [vinyl acetate
content: 7.5 mole %, MI: 2.5; m.p.: 79.degree. C., VSP: 62.degree.
C.] (a.sub.3), 20 wt. % of an elastomer [ethylene-.alpha.-olefin
thermoplastic elastomer, with the .alpha.-olefin being butene-1,
which is a random copolymer, butene-1 content: 12 mole %, MI: 4.0,
VSP: 50.degree. C., density 0.89 g/cm.sup.3 ] (b.sub.2) and 20 wt.
% of IPP (c.sub.1);
M.sub.411 : A mixture comprising 60 wt. % of EVA[Vinyl acetate
content: 2 mole %, MI: 0.6, m.p. 100.degree. C., VSP: 84.degree.
C.] (a.sub.4), 20 wt. % of the elastomer (b.sub.1) and 20 wt. % of
IPP (c.sub.1);
M.sub.511-3 : A mixture comprising 55 wt. % of EEA[ethyl acrylate
content: 5 mole %, MI: 1.5, m.p. 86.degree. C., VSP: 61.degree. C.]
(a.sub.5), 15 wt. % of the elastomer (b.sub.1), 15 wt. % of the IPP
(c.sub.1) and 15 wt. % of the PB-1 (c.sub.3);
M.sub.1-611 : A mixture comprising 35 wt. % of EVA (a.sub.2), 30
wt. % of L-LDPE [octene-1 is comonomer, 3.6% copolymerized, MI:
2.3, density: 0.915 g/cm3, m.p.: 116.degree.-120.degree. C. VSP:
98.degree. C.]: (a.sub.6), 20 wt. % of the elastomer (b.sub.1) and
15 wt. % of the IPP (c.sub.1);
M.sub.131 : A mixture comprising 70 wt. % of the EVA (a.sub.1), 15
wt. % of an elastomer (butyl rubber, VSP: 40.degree. C. or lower)
(b.sub.3) and 15 wt. % of the IPP (c.sub.1);
M.sub.141 : A mixture comprising 65 wt. % of the EVA (a.sub.1),
15wt. % of an elastomer (styrene-butadiene block copolymer
thermoplastic elastomer MI: 2.6) (b.sub.4) and 20 wt. % of the IPP
(c.sub.1);
M.sub.151 : A mixture comprising 70 wt. % of EVA (a.sub.1), 15 wt.
% of an elastomer (EPDM rubber, non-thermoplastic with 50 mole % of
ethylene content, Mooney viscosity of 40.degree. at 100.degree. C.)
(ML.sub.1+4) and 15 wt. % of IPP (c.sub.1);
M.sub.110 : A mixture comprising 75 wt. % of EVA (a.sub.1) and 25
wt. % of the elastomer (b.sub.1);
M.sub.011 : A mixture comprising 70 wt. % of the elasomer (b.sub.2)
and b 30 wt. % of the IPP (c.sub.1);
H.sub.0 : IPP [7 wt. % of ethylene randomly compolymerized, MFR: 6,
m.p. 135.degree. C., VSP: 125.degree. C.] (c.sub.0);
H.sub.1 : IPP (c.sub.1)=80 wt. %, PB-1 (c.sub.3)=20 wt %;
H.sub.2 : PB-1 (c.sub.3);
H.sub.3 : A mixture comprising 75 wt. % of IPP [ethylene content: 7
wt. %, MFR: 5, m.p.: 135.degree. C., VSP: 128.degree. C.] (c.sub.4)
and 25 wt. % of APP [atactic pp, VSP: 50.degree. C.];
H.sub.4 : A mixture comprising 80 wt. % of the IPP (c.sub.0) and 20
wt. % of an alicyclic saturated hydrocarbon resin;
a.sub.7 : EMMA [methyl methacrylate content: 6 mole %, MI: 2.5,
m.p.: 85.degree. C., VSP: 63.degree. C.];
a.sub.8 : Ionomer [ethylene-methacrylic acid copolymer,
Na-neutralized type, methacrylic acid content: 6.6 mole %, MI: 1.0,
neutralilzation: 25%, saponification: 50%, m.p. 80.degree. C., VSP:
64.degree. C.];
a.sub.9 : crystalline 1,2-polybutadiene (crystallinity: 25 %,
1,2-bonding: 92%, MFR (150.degree. C., load 2160 g): 3.0, density:
0.906 g, m.p.=80.degree. C., VSP=52.degree. C.); and
a.sub.10 : EVA [vinyl acetate content: 3.5 mole %, MI: 4.0, m.p.:
93.degree. C., VSP: 75.degree. C.]
All the films of Run No. 6-14 were satisfactory with good values of
physical properties. For comparative purpose, the raw film of Run
No. 9 was utilized, and it was subjected only to cold stretching in
the same manner as described above without post-treatment. The film
obtained had a thickness of 12.mu., longitudinal/transverse
stresses at 100% and 200% elongations of 1000/600 and 1700/1000
(g/cm-width), respectively. In stretch wrapping, creases were
generated abundantly, and the film could be elongated when drawn
strongly, but with breaking of tray. Thus, wrapping could not be
practiced well. However, this film could be sufficiently finished
within 1 to 2 seconds according to the shrink method with the hot
air of lower temperature of 80.degree. to 90.degree. C.
Each one of the films of Run No. 6-14 according to the present
invention, when wrapped by the stretch wrapping machine as
described above, could be wrapped and sealed excellently. The film
of control 4 was narrow in the proper temperature range as
85.degree. to 110.degree. C., whereby the temperature control was
slightly difficult and the dimensional stability of the film was
also poor (the film rolled up will be deformed with lapse of time).
All of the films of Run No. 6-14 were good in balance of heat
resistance, stretchability and sealability, compensating
synergistically mutually for antagonistic properties, with the
result that all the properties are satisfied.
EXAMPLE 3
According to the same procedure and and the same recipe of
additives as described in Example 2, with the use of a base layer
with a mixed composition of 100 pph of M.sub.111-3 with 5 pph of
AS.sub.1 for the second and fourth layers, H layer with a mixed
composition of the aforesaid c.sub.1 /c.sub.3 =7/3 (H.sub.5) for
the third layer and surface (S) layers of EEA (a.sub.5), LL
(a.sub.6), an ethylene-.alpha.-olefin thermoplastic elastomer
(b.sub.2), thermoplastic crystalline 1,2-polybutadiene (a.sub.9),
EMMA (a.sub.7) and Ionomer (a.sub.8) for the first and fifth
layers, five-layered raw films were prepared as Run No. 15, 16, 17,
18, 19 and 20 according to the combinations shown in Table 5. The
thicknesses of the respective layers of the first to fifth layers
were 7, 20, 8, 29 and 6.mu., respectively (70.mu. as the
total).
TABLE 5 ______________________________________ Run No. 15 16 17 18
19 20 ______________________________________ Layer struc- ture of
raw film First a.sub.5 a.sub.6 b.sub.2 a.sub.9 a.sub.7 a.sub.8
layer Second M.sub.111-3 layer Third H.sub.5 H.sub.5 H.sub.5
H.sub.5 H.sub.5 H.sub.5 layer Fourth M.sub.111-3 layer Fifth
a.sub.5 a.sub.6 b.sub.2 a.sub.9 a.sub.7 a.sub.8 layer
______________________________________
For the skin layers (the first and fifth layers), the same carrier
as in Example 1 was used as the tackifier (P agent) in combination
in an amount of 1.5 parts by weight with 100 parts by weight of
a.sub.6. Also, for the base layers (the second and fourth layers),
7 parts by weight of AS.sub.1 were employed as the carrier (AS
agent) in each Run No. 15, 17 and 18 per 100 parts by weight of the
base layer resin, while 7 parts by weight of the above APP
(AS.sub.2) were employed in Run No. 16, 10 parts of EVA (AS.sub.3)
with 40 wt. % of VAC in Run No. 19 and 5 parts by weight of the
resin of a.sub.9 (AS.sub.4) in Run No. 20.
Stretching could be practiced for each sample. The post-treatment
was applied at 50.degree. C. by permitting the film to shrink by
10% longitudinally and by 20% transversely. The stretching
temperatures were 46.degree., 59.degree., 42.degree., 40.degree.,
5.degree. and 51.degree. C., respectively. As Controls, the base
layers of the second and fourth layers were omitted in Run No.
15-20, and the third layer was made 30.mu. to obtain raw films of
100.mu.. When stretching was attempted to be carried out for these
films, no cold stretching could be effected, but the film was
punctured when it was expanded to a transverse expansion ratio
(BUR) of 1.5 to 2.0 by sealing the air into the bubble. Accordingly
the temperature was gradually elevated up to 92.degree. C., whereas
only Run No. 16 and 20 were unstable to result immediately in
puncture of the bubbles, and only small amount of films could be
obtained. However, these films were white in color, with haze
values of 20 and 15%, respectively, shrinkages at 80.degree. C.
being 4 and 6%, respectively and the tensile strengths at break as
low as 4.2 and 3.9 Kg/mm.sup.22. Thus, only films with low strength
could be obtained. Each of the films of Run No. 15-20 was about
10.mu. in thickness, and satisfied all the characteristics as films
for stetch wrapping, exhibiting respectively the Haze values of
0.7, 1.0, 0.6, 0.5, 0.9 and 0.4 (%); tensile strengths at break of
6.9, 8.0, 7.0, 7.3, 7.1 and 7.0 (Kg/mm.sup.2); elongations at break
(longitudinal/transverse) of 250/320, 270/360, 240/300, 250/300,
220/290 and 300/150 (%/%); stress at 100 %
(longitudinal/transverse): 310/130, 380/140, 230/110, 300/100,
270/120 and 300/150 (Kg/cm-width); stress at 200%
(longitudinal/transverse): 600/290, 650/310, 500/240, 570/220,
540/280 and 590/320 (Kg/cm-width); sealability being all
.circleincircle. , hand wrappability being all .circleincircle. ,
anti-fogging property being .circleincircle. , .circle. ,
.circleincircle. , .circleincircle. , .circleincircle. , .circle. ,
and surface staining resistance being all .circleincircle. .
The samples applied with no post-treatment could not afford
strength wrapping with an adequate stress, but creases could not
but remain on a part of the product to be wrapped, thus involving
the problem in wrapping finishing. Good looking finishing can be
accomplished by permitting the crease portion to be shrunk by
blowing hot air of 60.degree. to 80.degree. C. for 0.5 to 1
sec.
EXAMPLE 4
According to the same method as in Example 1, EVA (a.sub.2) was
employed for the first and fifth layers (S layer), a mixed
composition of M.sub.2111 containing 65 wt. % of EVA (a.sub.2), 20
wt. % of the elastomer (b.sub.1), 10 wt. % of IPP (C.sub.1), 5 wt.
% of AS.sub.1 as AS agent for the second and fourth layers (M
layer), and H.sub.5 comprising 70 wt. % of IPP (c.sub.1) and 30 wt.
% of PB-1 (c.sub.3) for the third layer (H layer). The M layers
contained anti-fogging agents of 2 wt. % as the total of a 2/1
mixture of glycerine monooleate and diglycerine monolaurate, and
the thickness ratios of the respective layers were controlled to
20% for S layers (each 10% for both the first and fifth layers),
65% for M layers (each 32.5% for both the second and fourth layers)
and 15% for H layer (the third layer) to obtain a raw film. The raw
film was stretched and subjected to the predetermined
post-treatments as shown in Table 6 to obtain stably and uniformly
films of about 10.mu. in thickness.
TABLE 6 ______________________________________ Run No. Control 21
22 23 24 5 6 7 8 ______________________________________ Treatment
45 50 60 70 20 50 110 110 tempera- ture (.degree.C.): Longitudi- 10
15 20 5 2 0 15 2 nal shrin- kage (%): Transverse 20 25 5 10 1 1 27
2 shrinkage (%): ______________________________________
TABLE 7
__________________________________________________________________________
Run No. Control Control Control Control Characteristics 21 22 23 24
5 6 7 8
__________________________________________________________________________
Haze % 0.5 0.8 0.7 0.9 0.6 0.8 7.5 13.0 Tensile strength (LD/TD)
9.7/7.8 8.5/7.9 7.7/8.0 9.5/7.6 13.5/9.5 12.4/9.4 3.1/2.8 2.6/2.5
Kg/mm.sup.2 Tensile elongation (LD/TD) 290/370 340/390 390/310
280/350 200/200 210/210 125/260 110/170 % Stress on 100% (LD/TD)
380/150 330/130 290/190 350/170 950/400 900/350 200/100 220/120
elongation Kg/cm width Stress on 200% (LD/TD) 600/230 550/220
560/310 630/290 1350/950 1200/820 --/160 --/(150) elongation Kg/cm
width Dart impact Kg .multidot. cm 19 20 18 16 23 20 4.7 3.0
strength Hand wrappability -- .circleincircle. .circleincircle.
.circleincircle. .circleincircle. x x x x
__________________________________________________________________________
Each of Run No. 21-24 involved no problems in transparency, film
strength, hand wrappability, sealability, and anti-fogging property
and each was excellent in wrappability by means of a stretch
wrapping machine. But the sample of Control 5, which corresponded
to the film of Blank with no application of the heat treatment of
the present invention, but too high in stress at 100% and 200%
elongation, and could hardly be stretched during stretch wrapping.
The same tendency was also observed for the film of Conrol 6. The
film of Control 6 was treated without shrinkage during heat
treatment and it was a film to which no effective elongation was
imparted. The film of Control 7 was treated by shrinkage under high
temperature, but its transparency was worsened to be lowered in
film strength, with the result that it was brittle, was readily
broken, and was accompanied with extreme thickness irregularity
(necking phenomenon) in the transverse direction when the film was
stretched, whereby the yield point occurred in the S-S curve. Thus,
this film could not be utilized for stretcth wrapping at all. The
same tendency was also observed for the sample of Control 8, which
was also brittle.
For samples of Run No. 21-24 and Control 5 and 6, the remaining low
temperature shrinkages at 80.degree. C. were from 20 to 45%, but
there was substantially no low temperature shrinkage remained in
Control 7 and 8. This means insufficiency in post-treatment of
Control 5 and 6. In short, Control 5 is the blank of the cold
stretched film itself, while Control 6 was subjected to heat
treatment without shrinkage. With such a treatment, no effective
elongation could be imparted to the film. The film of Control 7,
while it was subjected to shrinking treatment, suffers from loss of
good properties obtained by cold stretching because of too high
treatment temperature, whereby no synergistic effect could be
exhibited. Also, Control 8 became embrittled by a similar excessive
treatment.
EXAMPLE 5
According to the same procedure as in Example 4 as described above,
using the same layer constitution, composition and additives, and
also the compositions of M layers except for those indicaed in
Table 8 below, films were obtained and stetched. Post-treatment of
each film was conducted at a treatment temperature of 50.degree.
C., with longitudinal shrinkage of 10% and transverse shrinkage of
15%. M layers were constituted as follows to obtain films of about
10.mu., respectively.
TABLE 8 ______________________________________ Run No. 25 26 27 28
29 ______________________________________ Composi- tion of M layer
EVA (a.sub.2) 75 70 65 60 55 Elastomer 15 15 15 15 15 (b.sub.1) IPP
(c.sub.1) 10 10 10 10 10 AS agent -- 5 10 15 20 (AS.sub.1)
______________________________________
Each of these films was without any problem, having the properties
within the following ranges: haze of from 0.5 to 0.8%, tensile
strength at break (longitudinal/transverse) of (11 to 8)/(7 to 10)
(kg/mm.sup.2), elongation at break (longitudinal/transverse) of
(280 to 390)/(300 to 400) (%); falling dart impact strength of from
18 to 22 kg.cm, other characteristics being also within preferable
ranges. Also, the stresses at 200% elongation
(longitudinal/transverse) for Run No. 25-29 were 700/500, 550/360,
510/300, 460/250 and 430/200 (g/cm-width), while those at 100%
elongation were similarly in the same order 400/330, 320/230,
280/180, 260/150 and 250/130 (g/cm-width), respectively. In the
hand wrapping test, which was carried out by wrap stretching the
film in the transverse direction, the film of Run No. 25 was too
heavy and hardly stretched, but wrapping could be at last
accomplished. The films of Run No. 26-29, particularly 27-29, could
be wrapped smoothly.
As to the behaviour during cold stretching, the films of Run No.
26-28 tended to be particularly excellent in stability as compared
with Run No. 25. On the other hand, lowering in stress at
elongation (particularly stress at elongation within the range from
100 to 200 %) was found to be greater as the amount of AS agent was
increased fron Run No. 25 to Run No. 29.
EXAMPLE 6
According to the same procedure and the same compositions as
employed in Example 4, except for the combinations of the
respective layers in the raw films as shown in Table 9 and
incorporation of additives in both S layer and M layer, stretched
films were obtained. The conditions for post-treatment followed the
same conditions as in Run No. 22. As the result, films of about
11.mu. in thickness with stable working conditions could be
obtained.
These films, as written in the order of Run No. 30, 31, 32 and 33,
had the following physical properties, namely haze values (%) of
1.5, 0.6, 1.4 and 0.7; tensile strengths at break
(longitudinal/transverse=LD/TD) (kg/mm.sup.2) of 8.5/7.6, 9.0/8.0,
10.5/8.6, 11.0/8.2; elongations at break (%) (LD/TD): 235/350,
260/430, 275/340 and 250/320; Dart Impact Strength (Kg cm): 16.0,
17.5, 18.5 and 19.5; Stress at 200% elongation (g/cm-width)(LD/TD):
650/240, 560/230, 680/320 and 700/310; Stress at 100% elongation
(g/cm-width) (LD/TD): 370/190, 300/150, 400/220 and 430/210;
sealability being .circle. , .circleincircle. , .circle. , and
.circleincircle. , respectively; hand wrappability being all
.circleincircle. ; anti-fogging property being all .circleincircle.
; and surface staining resistance being .circle. , .circleincircle.
, .circle. , and .circleincircle. , respectively, each being
satisfactory and excellent as the film for stretch wrapping. On the
other hand, the film of Control 9 could be cold stretched smoothly,
but the film is prone to be curled as one sheet, and shrinked
creases are generated on the third layer (H layer) when subjected
to the post-treatment, whereby the haze value became 16% to give a
white impression, and the surface characteristics such as
sealability, anti-fogging property and tackiness were particularly
poor (in the case of employing the third layer), thus involving the
problem as the film for stretch wrapping.
* * * * *